diff --git a/runtime-pyside6/tests/plugins/natural/__init__.py b/runtime-pyside6/tests/plugins/natural/__init__.py
index 7a67869..4cfe179 100644
--- a/runtime-pyside6/tests/plugins/natural/__init__.py
+++ b/runtime-pyside6/tests/plugins/natural/__init__.py
@@ -18,4 +18,5 @@
 
 from .spy import Spy
 from .that import that
+from .interfaces.base import Assertion
 
diff --git a/runtime-pyside6/tests/plugins/natural/interfaces/assertion.py b/runtime-pyside6/tests/plugins/natural/interfaces/assertion.py
index f2504a2..e1e43c4 100644
--- a/runtime-pyside6/tests/plugins/natural/interfaces/assertion.py
+++ b/runtime-pyside6/tests/plugins/natural/interfaces/assertion.py
@@ -17,14 +17,23 @@
 """
 
 class Assertion(Exception):
-    def __init__(self, assertion: bool, message: str):
+    def __init__(self, assertion: bool, message: str, invert: bool):
         self.assertion = assertion
         self.message = message
+        self.invert = invert
+
+    def _invert_message(self):
+        for verb in ('is', 'was', 'has', 'have'):
+            for negative in ("n't", ' not', ' never', ' no'):
+                self.message = self.message.replace(f"{verb}{negative}", verb.upper())
 
     def __str__(self):
         return self.message
 
     def __bool__(self):
-        if not self.assertion:
+        if not self.invert and not self.assertion:
             raise self
-        return self.assertion
\ No newline at end of file
+        if self.invert and self.assertion:
+            self._invert_message()
+            raise self
+        return True # Raises otherwise.
\ No newline at end of file
diff --git a/runtime-pyside6/tests/plugins/natural/interfaces/base.py b/runtime-pyside6/tests/plugins/natural/interfaces/base.py
index 88b9e06..0786285 100644
--- a/runtime-pyside6/tests/plugins/natural/interfaces/base.py
+++ b/runtime-pyside6/tests/plugins/natural/interfaces/base.py
@@ -15,10 +15,10 @@
  *  You should have received a copy of the GNU General Public License
  *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
 """
-from typing import Self
+from typing import Self, Callable, Any
 
-from tests.plugins.natural.interfaces.assertion import Assertion
-from tests.plugins.natural.interfaces.utils import repr_
+from .assertion import Assertion
+from .utils import repr_
 
 
 class AssertionInterface:
@@ -26,15 +26,91 @@ class AssertionInterface:
     Most basic assertion interface.
     You probably want to use BaseAssertionInterface
     """
-    def __init__(self, value):
+
+    def __init__(self, value, parent: Self = None):
         self._value = value
+        self._parent = parent
+        if parent is None:
+            self.__not = False
 
     @property
-    def was(self) -> Self:
+    def _not(self) -> bool:
+        """
+        Internal state of whether the expression was negated.
+        Use "not_" to set it.
+        :return:
+        """
+        return self.__not if self._parent is None else self._parent._not
+
+    @_not.setter
+    def _not(self, value: bool):
+        if self._not is True:
+            raise RuntimeError("Cannot call is_not or was_not twice in the same statement.")
+        if self._parent is None:
+            self.__not = True
+        else:
+            self._parent._not = True
+
+    def instance_of(self, type_: type) -> Assertion:
+        """
+        Checks if the current value is equal to the provided value
+        """
+        value_type_name = type(self._value).__name__
+        if not isinstance(type_, type):
+            raise RuntimeError("Provided 'type' provided is not a class.")
+        return Assertion(
+            isinstance(self._value, type_),
+            f"The value ({value_type_name} {repr_(self._value)}) is not a {type_.__name__}.",
+            self._not
+        )
+
+    def __call__(self, condition: Callable[[Any], bool]) -> Assertion:
+        """
+        Apply condition to value that returns whether or not the value is valid.
+        """
+        return Assertion(
+            condition(self._value),
+            f"The value ({repr_(self._value)}) did not match given conditions.",
+            self._not
+        )
+
+    """
+    NOT Properties.
+    """
+
+    @property
+    def NOT(self) -> Self:
+        self._not = True
         return self
 
     @property
-    def be(self) -> Self:
+    def not_(self) -> Self:
+        self._not = True
+        return self
+
+    @property
+    def never(self) -> Self:
+        self._not = True
+        return self
+
+    """
+    Chain self properties to sound natural
+    """
+
+    @property
+    def that(self) -> Self:
+        return self
+
+    @property
+    def is_(self) -> Self:
+        return self
+
+    @property
+    def does(self) -> Self:
+        return self
+
+    @property
+    def was(self) -> Self:
         return self
 
     @property
@@ -57,31 +133,25 @@ class AssertionInterface:
     def an(self) -> Self:
         return self
 
-    def is_a(self, type_) -> Assertion:
-        """
-        Checks if the current value is equal to the provided value
-        """
-        value_type_name = type(self._value).__name__
-        return Assertion(isinstance(type_, type_), f"The value ({value_type_name} {repr_(self._value)}) is not a {type_.__name__}.")
-
-
 
 class EqualAssertionInterface(AssertionInterface):
     """
     Interface created for when its value should be checked for equality
     """
-    def __init__(self, value):
-        super().__init__(value)
+
+    def __init__(self, value, parent: AssertionInterface = None):
+        super().__init__(value, parent)
 
     def __call__(self, value) -> Assertion:
-        return Assertion(value == self._value, f"The value ({repr_(self._value)}) is not equal to {repr(value)}.")
-
-    def to(self, value) -> Self:
-        return self(value)
-
-    def of(self, value) -> Self:
-        return self(value)
+        return Assertion(
+            value == self._value,
+            f"The value {repr_(self._value)} is different from {repr(value)}.",
+            self._not
+        )
 
+    @property
+    def to(self) -> Self:
+        return self
 
 
 class BaseAssertionInterface(AssertionInterface):
@@ -91,11 +161,11 @@ class BaseAssertionInterface(AssertionInterface):
         """
         Checks if the current value is equal to the provided value
         """
-        return EqualAssertionInterface(self._value)
+        return EqualAssertionInterface(self._value, self)
 
     @property
-    def is_equal(self) -> EqualAssertionInterface:
+    def equal(self) -> EqualAssertionInterface:
         """
         Checks if the current value is equal to the provided value
         """
-        return self.equals
\ No newline at end of file
+        return self.equals
diff --git a/runtime-pyside6/tests/plugins/natural/interfaces/basic.py b/runtime-pyside6/tests/plugins/natural/interfaces/basic.py
index 1d5a878..25d45cb 100644
--- a/runtime-pyside6/tests/plugins/natural/interfaces/basic.py
+++ b/runtime-pyside6/tests/plugins/natural/interfaces/basic.py
@@ -15,47 +15,69 @@
  *  You should have received a copy of the GNU General Public License
  *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
 """
-from tests.plugins.natural.interfaces.assertion import Assertion
-from tests.plugins.natural.interfaces.base import BaseAssertionInterface
-from tests.plugins.natural.interfaces.int import IntInterface
-from tests.plugins.natural.interfaces.utils import repr_
+from .assertion import Assertion
+from .base import BaseAssertionInterface
+from .int import NumberInterface
+from .utils import repr_
 
 
 class FixedIteratorInterface(BaseAssertionInterface):
     @property
-    def length(self) -> IntInterface:
-        return IntInterface(len(self._value))
+    def length(self) -> NumberInterface:
+        return NumberInterface(len(self._value), self)
 
     def elements(self, *elements) -> Assertion:
-        tests = [elem for elem in elements if elem in self._value]
+        tests = [repr_(elem) for elem in elements if elem not in self._value]
         return Assertion(
             len(tests) == 0,
-            f"This value ({repr_(self._value)}) does not have elements ({repr_(tests)})"
+            f"This value ({repr_(self._value)}) does not have elements {', '.join(tests)}.",
+            self._not
         )
 
     def element(self, element) -> Assertion:
         return Assertion(
             element in self._value,
-            f"This value ({repr_(self._value)}) does not have element ({repr_(element)})"
+            f"This value ({repr_(self._value)}) does not have element {repr_(element)}.",
+            self._not
         )
 
     def contains(self, *elements) -> Assertion:
-        return self.elements(*elements)
+        """
+        Check if the element(s) are contained in the iterator.
+        """
+        if len(elements) == 1:
+            return self.element(elements[0])
+        else:
+            return self.elements(*elements)
+
+    def contain(self, *elements):
+        """
+        Check if the element(s) are contained in the iterator.
+        """
+        return self.contains(*elements)
+
 
 class BoolInterface(BaseAssertionInterface):
     @property
-    def is_true(self):
+    def true(self):
         return Assertion(
             self._value == True,
-            f"The value ({repr_(self._value)}) is not True."
+            f"The value ({repr_(self._value)}) is not True.",
+            self._not
         )
 
     @property
-    def is_false(self):
+    def false(self):
         return Assertion(
             self._value == False,
-            f"The value ({repr_(self._value)}) is not False."
+            f"The value ({repr_(self._value)}) is not False.",
+            self._not
         )
 
-class StringInterface(LengthInterface):
-    pass
\ No newline at end of file
+
+class StringInterface(FixedIteratorInterface):
+    pass
+
+
+class ListInterface(FixedIteratorInterface):
+    pass
diff --git a/runtime-pyside6/tests/plugins/natural/interfaces/int.py b/runtime-pyside6/tests/plugins/natural/interfaces/int.py
index aaf0511..7ac8871 100644
--- a/runtime-pyside6/tests/plugins/natural/interfaces/int.py
+++ b/runtime-pyside6/tests/plugins/natural/interfaces/int.py
@@ -15,17 +15,47 @@
  *  You should have received a copy of the GNU General Public License
  *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
 """
+from idlelib.configdialog import is_int
+from math import log10, floor, ceil
 from typing import Self
 
-from tests.plugins.natural.interfaces.assertion import Assertion
-from tests.plugins.natural.interfaces.base import BaseAssertionInterface, EqualAssertionInterface, AssertionInterface
-from tests.plugins.natural.interfaces.utils import repr_
+from .assertion import Assertion
+from .base import AssertionInterface
+from .utils import repr_
 
 
-class IntComparisonAssertionInterface(AssertionInterface):
-    def __init__(self, value):
-        super().__init__(value)
-        self._compare_to = None
+class NumberComparisonAssertionInterface(AssertionInterface):
+    def __init__(self, value, parent: AssertionInterface = None):
+        super().__init__(value, parent)
+        self._compare_stack = []
+
+    def _generate_compare_to(self) -> int:
+        """
+        The number generated by the comparison stack.
+        E.g. can parse one.hundred.million.and.thirty.three.thousand.and.twelve.hundred.and.seven
+                    as ['one', 'hundred', 'million', 'thirty', 'three', 'thousand', 'twelve', 'hundred', 'seven']
+         which results 100,034,207
+        """
+        minus = len(self._compare_stack) > 0 and self._compare_stack[0] == -1
+        if len(self._compare_stack) < (2 if minus else 1):
+            raise RuntimeError("No number to compare the value to provided.")
+        if minus:
+            self._compare_stack.pop(0)
+        # Compute the number
+        add_stack = [self._compare_stack.pop(0)]
+        for element in self._compare_stack:
+            last_power = floor(log10(abs(add_stack[-1])))
+            current_power = floor(log10(abs(element)))
+            if last_power < current_power:  # E.g. one hundred
+                add_stack[-1] *= element
+            elif last_power == 1 and current_power == 0:  # E.g thirty four
+                add_stack[-1] += element
+            elif last_power > current_power:  # E.g a hundred and five
+                add_stack.append(element)
+            else:
+                raise RuntimeError(f"Cannot chain two numbers with the same power ({add_stack[-1]} => {element}.")
+        total = sum(add_stack)
+        return -total if minus else total
 
     def _compare(self) -> Assertion:
         raise RuntimeError(f"No comparison method defined in {type(self).__name__}.")
@@ -34,23 +64,26 @@ class IntComparisonAssertionInterface(AssertionInterface):
         return bool(self._compare())
 
     def __call__(self, compare_to: int) -> Self:
-        if self._compare_to is None:
-            self._compare_to = int(compare_to)
-        else:
-            self._compare_to *= int(compare_to)
+        if type(compare_to) not in (float, int):
+            raise RuntimeError(f"Cannot compare number ({self._value}) to non number ({repr_(compare_to)}).")
+        self._compare_stack.append(compare_to)
+        return self
+
+    """
+    Chain self properties
+    """
+
+    @property
+    def and_(self) -> Self:
         return self
 
     @property
-    def time(self) -> Self:
+    def AND(self) -> Self:
         return self
 
-    @property
-    def times(self) -> Self:
-        return self
-
-    @property
-    def never(self) -> Self:
-        return self(0)
+    """
+    Number shorthands
+    """
 
     @property
     def once(self) -> Self:
@@ -64,6 +97,10 @@ class IntComparisonAssertionInterface(AssertionInterface):
     def thrice(self) -> Self:
         return self(3)
 
+    @property
+    def minus(self) -> Self:
+        return self(-1)
+
     @property
     def zero(self) -> Self:
         return self(0)
@@ -108,6 +145,42 @@ class IntComparisonAssertionInterface(AssertionInterface):
     def ten(self) -> Self:
         return self(10)
 
+    @property
+    def eleven(self) -> Self:
+        return self(11)
+
+    @property
+    def twelve(self) -> Self:
+        return self(12)
+
+    @property
+    def thirteen(self) -> Self:
+        return self(13)
+
+    @property
+    def fourteen(self) -> Self:
+        return self(14)
+
+    @property
+    def fifteen(self) -> Self:
+        return self(15)
+
+    @property
+    def sixteen(self) -> Self:
+        return self(16)
+
+    @property
+    def seventeen(self) -> Self:
+        return self(17)
+
+    @property
+    def eighteen(self) -> Self:
+        return self(18)
+
+    @property
+    def nineteen(self) -> Self:
+        return self(19)
+
     @property
     def twenty(self) -> Self:
         return self(20)
@@ -134,11 +207,11 @@ class IntComparisonAssertionInterface(AssertionInterface):
 
     @property
     def eighty(self) -> Self:
-        return self(70)
+        return self(80)
 
     @property
     def ninety(self) -> Self:
-        return self(70)
+        return self(90)
 
     @property
     def hundred(self) -> Self:
@@ -157,73 +230,92 @@ class IntComparisonAssertionInterface(AssertionInterface):
         return self(1_000_000_000)
 
 
-class LessThanComparisonInterface(IntComparisonAssertionInterface):
+class LessThanComparisonInterface(NumberComparisonAssertionInterface):
     def _compare(self) -> Assertion:
+        compare = self._generate_compare_to()
         return Assertion(
-            self._value < self._compare_to,
-            f"The value ({repr_(self._value)}) is not less than to {repr_(self._compare_to)}."
+            self._value < compare,
+            f"The value ({repr_(self._value)}) is not less than to {repr_(compare)}.",
+            self._not
         )
 
-class MoreThanComparisonInterface(IntComparisonAssertionInterface):
+
+class MoreThanComparisonInterface(NumberComparisonAssertionInterface):
     def _compare(self) -> Assertion:
+        compare = self._generate_compare_to()
         return Assertion(
-            self._value > self._compare_to,
-            f"The value ({repr_(self._value)}) is not more than to {repr_(self._compare_to)}."
+            self._value > compare,
+            f"The value ({repr_(self._value)}) is not more than to {repr_(compare)}.",
+            self._not
         )
 
-class AtLeastComparisonInterface(IntComparisonAssertionInterface):
+
+class AtLeastComparisonInterface(NumberComparisonAssertionInterface):
     def _compare(self) -> Assertion:
+        compare = self._generate_compare_to()
         return Assertion(
-            self._value >= self._compare_to,
-            f"The value ({repr_(self._value)}) is not at least to {repr_(self._compare_to)}."
+            self._value >= compare,
+            f"The value ({repr_(self._value)}) is not at least to {repr_(compare)}.",
+            self._not
         )
 
-class AtMostComparisonInterface(IntComparisonAssertionInterface):
+
+class AtMostComparisonInterface(NumberComparisonAssertionInterface):
     def _compare(self) -> Assertion:
+        compare = self._generate_compare_to()
         return Assertion(
-            self._value <= self._compare_to,
-            f"The value ({repr_(self._value)}) is not at least to {repr_(self._compare_to)}."
+            self._value <= compare,
+            f"The value ({repr_(self._value)}) is not at least to {repr_(compare)}.",
+            self._not
         )
 
-class EqualComparisonInterface(IntComparisonAssertionInterface):
+
+class EqualComparisonInterface(NumberComparisonAssertionInterface):
     def _compare(self) -> Assertion:
+        compare = self._generate_compare_to()
         return Assertion(
-            self._value == self._compare_to,
-            f"The value ({repr_(self._value)}) is not equal to {repr_(self._compare_to)}."
+            self._value == compare,
+            f"The value ({repr_(self._value)}) is not equal to {repr_(compare)}.",
+            self._not
         )
 
+    @property
     def to(self) -> Self:
         return self
 
-    def of(self) -> Self:
-        return self
 
-
-class IntInterface(AssertionInterface):
-    def less_than(self) -> LessThanComparisonInterface:
-        return LessThanComparisonInterface(self._value)
-
-    @property
-    def more_than(self) -> MoreThanComparisonInterface:
-        return MoreThanComparisonInterface(self._value)
-
-    @property
-    def at_least(self) -> AtLeastComparisonInterface:
-        return AtLeastComparisonInterface(self._value)
-
-    @property
-    def at_most(self) -> AtMostComparisonInterface:
-        return AtMostComparisonInterface(self._value)
+class NumberInterface(AssertionInterface):
+    def __call__(self, value):
+        return EqualComparisonInterface(self._value, self)(value)
 
     @property
     def equals(self) -> EqualComparisonInterface:
-        return EqualComparisonInterface(self._value)
+        return EqualComparisonInterface(self._value, self)
 
     @property
-    def is_equal(self) -> EqualComparisonInterface:
-        return EqualComparisonInterface(self._value)
+    def equal(self) -> EqualComparisonInterface:
+        return EqualComparisonInterface(self._value, self)
 
     @property
     def exactly(self) -> EqualComparisonInterface:
-        return EqualComparisonInterface(self._value)
+        return EqualComparisonInterface(self._value, self)
 
+    @property
+    def of(self) -> EqualComparisonInterface:
+        return EqualComparisonInterface(self._value, self)
+
+    @property
+    def less_than(self) -> LessThanComparisonInterface:
+        return LessThanComparisonInterface(self._value, self)
+
+    @property
+    def more_than(self) -> MoreThanComparisonInterface:
+        return MoreThanComparisonInterface(self._value, self)
+
+    @property
+    def at_least(self) -> AtLeastComparisonInterface:
+        return AtLeastComparisonInterface(self._value, self)
+
+    @property
+    def at_most(self) -> AtMostComparisonInterface:
+        return AtMostComparisonInterface(self._value, self)
diff --git a/runtime-pyside6/tests/plugins/natural/interfaces/spy.py b/runtime-pyside6/tests/plugins/natural/interfaces/spy.py
new file mode 100644
index 0000000..59b09d3
--- /dev/null
+++ b/runtime-pyside6/tests/plugins/natural/interfaces/spy.py
@@ -0,0 +1,218 @@
+"""
+ *  LogarithmPlotter - 2D plotter software to make BODE plots, sequences and distribution functions.
+ *  Copyright (C) 2021-2024  Ad5001
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""
+
+from typing import Callable, Self
+
+from .base import Assertion, repr_, AssertionInterface
+from .int import NumberComparisonAssertionInterface
+
+PRINT_PREFIX = (" " * 24)
+
+
+class SpyAssertion(Assertion):
+    def __init__(self, assertion: bool, message: str, calls: list, invert: bool):
+        super().__init__(assertion, message + "\n", invert)
+        if len(calls) > 0:
+            self.message += self.render_calls(calls)
+        else:
+            self.message += f"{PRINT_PREFIX}0 registered calls."
+
+    def render_calls(self, calls):
+        lines = [f"{PRINT_PREFIX}{len(calls)} registered call(s):"]
+        for call in calls:
+            repr_args = [repr_(arg) for arg in call[0]]
+            repr_kwargs = [f"{key}={repr_(arg)}" for key, arg in call[1].items()]
+            lines.append(f"  - {', '.join([*repr_args, *repr_kwargs])}")
+        return ("\n" + PRINT_PREFIX).join(lines)
+
+
+class Methods:
+    AT_LEAST_ONCE = "AT_LEAST_ONCE"
+    EXACTLY = "EXACTLY"
+    AT_LEAST = "AT_LEAST"
+    AT_MOST = "AT_MOST"
+    MORE_THAN = "MORE_THAN"
+    LESS_THAN = "LESS_THAN"
+
+
+class CalledInterface(NumberComparisonAssertionInterface):
+    """
+    Internal class generated by Spy.called.
+    """
+
+    def __init__(self, calls: list[tuple[list, dict]], parent: AssertionInterface):
+        super().__init__(len(calls), parent)
+        self.__calls = calls
+        self.__method = Methods.AT_LEAST_ONCE
+
+    def __apply_method(self, calls):
+        required = None if self._compare_stack == [] else self._generate_compare_to()
+        calls_count = len(calls)
+        match self.__method:
+            case Methods.AT_LEAST_ONCE:
+                compare = len(calls) >= 1
+                error = f"Method was not called"
+            case Methods.EXACTLY:
+                compare = len(calls) == required
+                error = f"Method was not called {required} times ({required} != {calls_count})"
+            case Methods.AT_LEAST:
+                compare = len(calls) >= required
+                error = f"Method was not called at least {required} times ({required} >= {calls_count})"
+            case Methods.AT_MOST:
+                compare = len(calls) <= required
+                error = f"Method was not called at most {required} times ({required} <= {calls_count})"
+            case Methods.MORE_THAN:
+                compare = len(calls) > required
+                error = f"Method was not called more than {required} times ({required} > {calls_count})"
+            case Methods.LESS_THAN:
+                compare = len(calls) < required
+                error = f"Method was not called less than {required} times ({required} < {calls_count})"
+            case _:
+                raise RuntimeError(f"Unknown method {self.__method}.")
+        return compare, error
+
+    def __bool__(self) -> bool:
+        """
+        Converts to boolean on assertion.
+        """
+        compare, error = self.__apply_method(self.__calls)
+        return bool(SpyAssertion(compare, error + ".", self.__calls, self._not))
+
+    """
+    Chaining methods
+    """
+
+    def __call__(self, compare_to: int) -> Self:
+        super().__call__(compare_to)
+        if self.__method == Methods.AT_LEAST_ONCE:
+            self.__method = Methods.EXACTLY
+        return self
+
+    @property
+    def at_least(self) -> Self:
+        if self.__method == Methods.AT_LEAST_ONCE:
+            self.__method = Methods.AT_LEAST
+        else:
+            raise RuntimeError(f"Cannot redefine method from {self.__method} to {Methods.AT_MOST}")
+        return self
+
+    @property
+    def at_most(self) -> Self:
+        if self.__method == Methods.AT_LEAST_ONCE:
+            self.__method = Methods.AT_MOST
+        else:
+            raise RuntimeError(f"Cannot redefine method from {self.__method} to {Methods.AT_MOST}")
+        return self
+
+    @property
+    def more_than(self) -> Self:
+        if self.__method == Methods.AT_LEAST_ONCE:
+            self.__method = Methods.MORE_THAN
+        else:
+            raise RuntimeError(f"Cannot redefine method from {self.__method} to {Methods.MORE_THAN}")
+        return self
+
+    @property
+    def less_than(self) -> Self:
+        if self.__method == Methods.AT_LEAST_ONCE:
+            self.__method = Methods.LESS_THAN
+        else:
+            raise RuntimeError(f"Cannot redefine method from {self.__method} to {Methods.LESS_THAN}")
+        return self
+
+    @property
+    def time(self) -> Self:
+        return self
+
+    @property
+    def times(self) -> Self:
+        return self
+
+    """
+    Class properties.
+    """
+
+    def __match_calls_for_condition(self, condition: Callable[[list, dict], bool]) -> tuple[bool, str]:
+        calls = []
+        for call in self.__calls:
+            if condition(call[0], call[1]):
+                calls.append(call)
+        compare, error = self.__apply_method(calls)
+        return compare, error
+
+    def with_arguments(self, *args, **kwargs) -> SpyAssertion:
+        """
+        Checks if the Spy has been called the given number of times
+        with at least the given arguments.
+        """
+
+        def some_args_matched(a, kw):
+            args_matched = all((
+                arg in a
+                for arg in args
+            ))
+            kwargs_matched = all((
+                key in kw and kw[key] == arg
+                for key, arg in kwargs.items()
+            ))
+            return args_matched and kwargs_matched
+
+        compare, error = self.__match_calls_for_condition(some_args_matched)
+        repr_args = ', '.join([repr(arg) for arg in args])
+        repr_kwargs = ', '.join([f"{key}={repr(arg)}" for key, arg in kwargs.items()])
+        msg = f"{error} with arguments ({repr_args}) and keyword arguments ({repr_kwargs})."
+        return SpyAssertion(compare, msg, self.__calls, self._not)
+
+    def with_arguments_matching(self, test_condition: Callable[[list, dict], bool]) -> SpyAssertion:
+        """
+        Checks if the Spy has been called the given number of times
+        with arguments matching the given conditions.
+        """
+        compare, error = self.__match_calls_for_condition(test_condition)
+        msg = f"{error} with arguments matching given conditions."
+        return SpyAssertion(compare, msg, self.__calls, self._not)
+
+    def with_exact_arguments(self, *args, **kwargs) -> SpyAssertion:
+        """
+        Checks if the Spy has been called the given number of times
+        with all the given arguments.
+        """
+        compare, error = self.__match_calls_for_condition(lambda a, kw: a == args and kw == kwargs)
+        repr_args = ', '.join([repr(arg) for arg in args])
+        repr_kwargs = ', '.join([f"{key}={repr(arg)}" for key, arg in kwargs.items()])
+        msg = f"{error} with exact arguments ({repr_args}) and keyword arguments ({repr_kwargs})."
+        return SpyAssertion(compare, msg, self.__calls, self._not)
+
+    def with_no_argument(self) -> SpyAssertion:
+        """
+        Checks if the Spy has been called the given number of times
+        with all the given arguments.
+        """
+        compare, error = self.__match_calls_for_condition(lambda a, kw: len(a) == 0 and len(kw) == 0)
+        return SpyAssertion(compare, f"{error} with no arguments.", self.__calls, self._not)
+
+
+class SpyAssertionInterface(AssertionInterface):
+
+    @property
+    def called(self) -> CalledInterface:
+        """
+        Returns a boolean-able interface to check conditions for a given number of
+        time the spy was called.
+        """
+        return CalledInterface(self._value.calls, self)
diff --git a/runtime-pyside6/tests/plugins/natural/spy.py b/runtime-pyside6/tests/plugins/natural/spy.py
index 1d77629..fbd8e17 100644
--- a/runtime-pyside6/tests/plugins/natural/spy.py
+++ b/runtime-pyside6/tests/plugins/natural/spy.py
@@ -15,207 +15,15 @@
  *  You should have received a copy of the GNU General Public License
  *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
 """
-from typing import Callable, Self
-
-from tests.plugins.natural.interfaces.base import Assertion, repr_, AssertionInterface
-from tests.plugins.natural.interfaces.int import IntComparisonAssertionInterface
-
-PRINT_PREFIX = (" " * 24)
+from typing import Callable
 
 
-class SpyAssertion(Assertion):
-    def __init__(self, assertion: bool, message: str, calls: list):
-        super().__init__(assertion, message + "\n")
-        if len(calls) > 0:
-            self.message += self.render_calls(calls)
-        else:
-            self.message += f"{PRINT_PREFIX}0 registered calls."
-
-
-    def render_calls(self, calls):
-        lines = [f"{PRINT_PREFIX}{len(calls)} registered call(s):"]
-        for call in calls:
-            repr_args = [repr_(arg) for arg in call[0]]
-            repr_kwargs = [f"{key}={repr_(arg)}" for key, arg in call[1].items()]
-            lines.append(f"  - {', '.join([*repr_args, *repr_kwargs])}")
-        return ("\n" + PRINT_PREFIX).join(lines)
-
-
-class Methods:
-    AT_LEAST_ONCE = "AT_LEAST_ONCE"
-    EXACTLY = "EXACTLY"
-    AT_LEAST = "AT_LEAST"
-    AT_MOST = "AT_MOST"
-    MORE_THAN = "AT_LEAST"
-    LESS_THAN = "AT_MOST"
-
-
-class CalledInterface(IntComparisonAssertionInterface):
-    """
-    Internal class generated by Spy.was_called.
-    """
-
-    def __init__(self, calls: list[tuple[list, dict]]):
-        super().__init__(len(calls))
-        self.__calls = calls
-        self.__method = Methods.AT_LEAST_ONCE
-
-    def __apply_method(self, calls):
-        required = self._compare_to
-        calls_count = len(calls)
-        match self.__method:
-            case Methods.AT_LEAST_ONCE:
-                compare = len(calls) >= 1
-                error = "Method was not called"
-            case Methods.EXACTLY:
-                compare = len(calls) == required
-                error = f"Method was not called {required} times ({required} != {calls_count})"
-            case Methods.AT_LEAST:
-                compare = len(calls) >= required
-                error = f"Method was not called at least {required} times ({required} > {calls_count})"
-            case Methods.AT_MOST:
-                compare = len(calls) <= required
-                error = f"Method was not called at most {required} times ({required} < {calls_count})"
-            case Methods.MORE_THAN:
-                compare = len(calls) > required
-                error = f"Method was not called more than {required} times ({required} >= {calls_count})"
-            case Methods.LESS_THAN:
-                compare = len(calls) < required
-                error = f"Method was not called less than {required} times ({required} <= {calls_count})"
-            case _:
-                raise RuntimeError(f"Unknown method {self.__method}.")
-        return compare, error
-
-    def __bool__(self) -> bool:
-        """
-        Converts to boolean on assertion.
-        """
-        compare, error = self.__apply_method(self.__calls)
-        return bool(SpyAssertion(compare, error + ".", self.__calls))
-
-    """
-    Chaining methods
-    """
-
-    def __call__(self, *args, **kwargs) -> Self:
-        if len(args) != 1:
-            raise RuntimeError("Cannot call called interface with more than one argument.")
-        self._compare_to = int(args[0])
-        if self.__method == Methods.AT_LEAST_ONCE:
-            self.__method = Methods.EXACTLY
-        return self
-
-    @property
-    def at_least(self) -> Self:
-        if self.__method == Methods.AT_LEAST_ONCE:
-            self.__method = Methods.AT_LEAST
-        else:
-            raise RuntimeError(f"Cannot redefine method from {self.__method} to {Methods.AT_MOST}")
-        return self
-
-    @property
-    def at_most(self) -> Self:
-        if self.__method == Methods.AT_LEAST_ONCE:
-            self.__method = Methods.AT_MOST
-        else:
-            raise RuntimeError(f"Cannot redefine method from {self.__method} to {Methods.AT_MOST}")
-        return self
-
-    @property
-    def more_than(self) -> Self:
-        if self.__method == Methods.AT_LEAST_ONCE:
-            self.__method = Methods.MORE_THAN
-        else:
-            raise RuntimeError(f"Cannot redefine method from {self.__method} to {Methods.MORE_THAN}")
-        return self
-
-    @property
-    def less_than(self) -> Self:
-        if self.__method == Methods.AT_LEAST_ONCE:
-            self.__method = Methods.LESS_THAN
-        else:
-            raise RuntimeError(f"Cannot redefine method from {self.__method} to {Methods.LESS_THAN}")
-        return self
-
-    @property
-    def time(self) -> Self:
-        return self
-
-    @property
-    def times(self) -> Self:
-        return self
-
-    """
-    Class properties.
-    """
-
-    def __match_calls_for_condition(self, condition: Callable[[list, dict], bool]) -> tuple[bool, str]:
-        calls = []
-        for call in self.__calls:
-            if condition(call[0], call[1]):
-                calls.append(call)
-        compare, error = self.__apply_method(calls)
-        return compare, error
-
-    def with_arguments(self, *args, **kwargs) -> SpyAssertion:
-        """
-        Checks if the Spy has been called the given number of times
-        with at least the given arguments.
-        """
-
-        def some_args_matched(a, kw):
-            args_matched = all((
-                arg in a
-                for arg in args
-            ))
-            kwargs_matched = all((
-                key in kw and kw[key] == arg
-                for key, arg in kwargs.items()
-            ))
-            return args_matched and kwargs_matched
-
-        compare, error = self.__match_calls_for_condition(some_args_matched)
-        repr_args = ', '.join([repr(arg) for arg in args])
-        repr_kwargs = ', '.join([f"{key}={repr(arg)}" for key, arg in kwargs.items()])
-        msg = f"{error} with arguments ({repr_args}) and keyword arguments ({repr_kwargs})."
-        return SpyAssertion(compare, msg, self.__calls)
-
-    def with_arguments_matching(self, test_condition: Callable[[list, dict], bool]) -> SpyAssertion:
-        """
-        Checks if the Spy has been called the given number of times
-        with arguments matching the given conditions.
-        """
-        compare, error = self.__match_calls_for_condition(test_condition)
-        msg = f"{error} with arguments matching given conditions."
-        return SpyAssertion(compare, msg, self.__calls)
-
-    def with_exact_arguments(self, *args, **kwargs) -> SpyAssertion:
-        """
-        Checks if the Spy has been called the given number of times
-        with all the given arguments.
-        """
-        compare, error = self.__match_calls_for_condition(lambda a, kw: a == args and kw == kwargs)
-        repr_args = ', '.join([repr(arg) for arg in args])
-        repr_kwargs = ', '.join([f"{key}={repr(arg)}" for key, arg in kwargs.items()])
-        msg = f"{error} with exact arguments ({repr_args}) and keyword arguments ({repr_kwargs})."
-        return SpyAssertion(compare, msg, self.__calls)
-
-    def with_no_argument(self) -> SpyAssertion:
-        """
-        Checks if the Spy has been called the given number of times
-        with all the given arguments.
-        """
-        compare, error = self.__match_calls_for_condition(lambda a, kw: len(a) == 0 and len(kw) == 0)
-        return SpyAssertion(compare, f"{error} with no arguments.", self.__calls)
-
-
-class Spy(AssertionInterface):
+class Spy:
     """
     Class to spy into method calls with natural language expressions.
     """
 
     def __init__(self, function: Callable = None):
-        super().__init__(function)
         self.function = function
         self.calls = []
 
@@ -223,20 +31,3 @@ class Spy(AssertionInterface):
         self.calls.append((args, kwargs))
         if self.function is not None:
             self.function(*args, **kwargs)
-
-    @property
-    def called(self) -> CalledInterface:
-        """
-        Returns a boolean-able interface to check conditions for a given number of
-        time the spy was called.
-        """
-        return CalledInterface(self.calls)
-
-    @property
-    def not_called(self) -> CalledInterface:
-        """
-        Returns a boolean-able interface to check that conditions were never fulfilled
-        in the times the spy was called.
-        """
-        ret = CalledInterface(self.calls)
-        return ret(0)
diff --git a/runtime-pyside6/tests/plugins/natural/that.py b/runtime-pyside6/tests/plugins/natural/that.py
index 38ee54e..05a3e31 100644
--- a/runtime-pyside6/tests/plugins/natural/that.py
+++ b/runtime-pyside6/tests/plugins/natural/that.py
@@ -17,17 +17,21 @@
 """
 from typing import overload, Generic, TypeVar
 
-from .spy import Spy
+from . import Spy
 from .interfaces.base import AssertionInterface, BaseAssertionInterface
-from .interfaces.basic import StringInterface, BoolInterface
-from .interfaces.int import IntInterface
+from .interfaces.basic import StringInterface, BoolInterface, ListInterface
+from .interfaces.int import NumberInterface
+from .interfaces.spy import SpyAssertionInterface
 
 Interface = TypeVar("Interface", bound=AssertionInterface)
 
 MATCHES = [
     (str, StringInterface),
-    (int, IntInterface),
     (bool, BoolInterface),
+    (int, NumberInterface),
+    (float, NumberInterface),
+    (list, ListInterface),
+    (Spy, SpyAssertionInterface)
 ]
 
 
@@ -36,11 +40,18 @@ def that(value: str) -> StringInterface: ...
 
 
 @overload
-def that(value: int) -> IntInterface: ...
+def that(value: bool) -> BoolInterface: ...
 
 
 @overload
-def that(value: bool) -> BoolInterface: ...
+def that(value: int) -> NumberInterface: ...
+
+
+@overload
+def that(value: float) -> NumberInterface: ...
+
+@overload
+def that(value: Spy) -> SpyAssertionInterface: ...
 
 
 @overload
diff --git a/runtime-pyside6/tests/plugins/tests/__init__.py b/runtime-pyside6/tests/plugins/tests/__init__.py
new file mode 100644
index 0000000..e69de29
diff --git a/runtime-pyside6/tests/plugins/tests/test_natural.py b/runtime-pyside6/tests/plugins/tests/test_natural.py
new file mode 100644
index 0000000..d7fe26c
--- /dev/null
+++ b/runtime-pyside6/tests/plugins/tests/test_natural.py
@@ -0,0 +1,217 @@
+"""
+ *  LogarithmPlotter - 2D plotter software to make BODE plots, sequences and distribution functions.
+ *  Copyright (C) 2021-2024  Ad5001
+ *
+ *  This program is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  This program is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
+"""
+import pytest
+
+from ..natural import that, Assertion, Spy
+
+
+def test_string():
+    assert that("QWERTY").is_.an.instance_of(str)
+    assert that("QWERTY").is_.not_.an.instance_of(int)
+    assert that("QWERTY").is_.equal.to("QWERTY")
+    assert that("QWERTY").is_.NOT.equal.to("QWERTYUIOP")
+    assert that("QWERTY").is_.NOT.equal.to(3)
+    assert that("QWERTY").has.a.length.of(6)
+    assert that("QWERTY").does.NOT.have.a.length.of(7)
+    assert that("QWERTY").has.a.length.that.is_.NOT(5)
+    assert that("QWERTY").contains("WER")
+    assert that("QWERTY").contains("WER", "TY")
+    assert that("QWERTY").does.not_.contain("AZERTY")
+    with pytest.raises(Assertion):
+        assert that("QWERTY").is_.an.instance_of(int)
+    with pytest.raises(Assertion):
+        assert that("QWERTY").is_.equal.to(False)
+    with pytest.raises(Assertion):
+        assert that("QWERTY").has.a.length.of(1)
+    with pytest.raises(Assertion):
+        assert that("QWERTY").contains("AZERTY")
+    with pytest.raises(Assertion):
+        assert that("QWERTY").does.NOT.contain("QWE")
+
+def test_bool():
+    assert that(True).is_.an.instance_of(bool)
+    assert that(True).is_.an.instance_of(int)
+    assert that(True).is_.NOT.an.instance_of(str)
+    assert that(True).equals(True)
+    assert that(True).is_.true
+    assert that(True).is_.NOT.false
+    assert that(False).is_.equal.to(False)
+    assert that(False).is_.false
+    assert that(False).is_.NOT.true
+    with pytest.raises(Assertion):
+        assert that(True).is_.false
+    with pytest.raises(Assertion):
+        assert that(True).is_.NOT.true
+
+def test_int():
+    assert that(2).is_.an.instance_of(int)
+    assert that(2).is_.NOT.an.instance_of(bool)
+    assert that(2).is_.NOT.an.instance_of(str)
+    assert that(2).is_.more_than(1)
+    assert that(2).is_.NOT.less_than(1)
+    assert that(2).is_.less_than(3)
+    assert that(2).is_.NOT.more_than(3)
+    assert that(2).is_.at_least(1)
+    assert that(2).is_.NOT.at_most(1)
+    assert that(2).is_.at_most(3)
+    assert that(2).is_.NOT.at_least(3)
+    assert that(2).is_.at_most(2)
+    assert that(2).is_.at_least(2)
+    # Equality
+    assert that(2).is_(2)
+    assert that(2).was(2)
+    assert that(2).is_.exactly(2)
+    assert that(2).is_.equal.to(2)
+    assert that(2).equals(2)
+    assert that(2).is_.NOT(3)
+    assert that(2).does.NOT.equal(3)
+
+def test_int_shorthands():
+    # Direct numbers
+    assert that(0).equals.zero
+    assert that(1).equals.one
+    assert that(2).equals.two
+    assert that(3).equals.three
+    assert that(4).equals.four
+    assert that(5).equals.five
+    assert that(6).equals.six
+    assert that(7).equals.seven
+    assert that(8).equals.eight
+    assert that(9).equals.nine
+    assert that(10).equals.ten
+    assert that(11).equals.eleven
+    assert that(12).equals.twelve
+    assert that(13).equals.thirteen
+    assert that(14).equals.fourteen
+    assert that(15).equals.fifteen
+    assert that(16).equals.sixteen
+    assert that(17).equals.seventeen
+    assert that(18).equals.eighteen
+    assert that(19).equals.nineteen
+    assert that(20).equals.twenty
+    assert that(30).equals.thirty
+    assert that(40).equals.forty
+    assert that(50).equals.fifty
+    assert that(60).equals.sixty
+    assert that(70).equals.seventy
+    assert that(80).equals.eighty
+    assert that(90).equals.ninety
+    assert that(100).equals.a.hundred
+    assert that(1000).equals.a.thousand
+    assert that(1_000_000).equals.a.million
+
+def test_add_natural_complex():
+    # Test composed
+    assert that(34).equals.thirty.four
+    assert that(-34).equals.minus.thirty.four
+    assert that(100_033_207).equals.one.hundred.million.AND.thirty.three.thousand.AND.two.hundred.AND.seven
+    assert that(-1_200_033_207).equals.minus.one.billion.AND.two.hundred.million.AND.thirty.three.thousand.AND.two.hundred.AND.seven
+    assert that(7890).equals.seven.thousand.eight.hundred.and_.ninety
+    assert that(7890).equals.seventy.eight.hundred.and_.ninety
+    assert that(7890).equals(78)(100)(90)
+    with pytest.raises(RuntimeError):
+        assert that(1_000_000).equals.a.thousand.thousand
+    with pytest.raises(RuntimeError):
+        assert that(600).equals.one.twenty.thirty
+    with pytest.raises(RuntimeError):
+        assert that(2).equals
+    with pytest.raises(RuntimeError):
+        assert that(2).equals.one.minus.two
+
+def test_spy():
+    spy = Spy()
+    assert that(spy).is_.an.instance_of(Spy)
+    assert that(spy).is_(callable)
+    # Check calls
+    assert that(spy).was.never.called
+    assert that(spy).was.called.zero.times
+    spy(30, arg="string")
+    assert that(spy).was.called
+    assert that(spy).was.called.once
+    assert that(spy).was.called.one.time
+    assert that(spy).was.NOT.called.more_than.once
+    assert that(spy).was.called.with_arguments(30)
+    assert that(spy).was.called.with_arguments_matching(lambda args, kwargs: len(args) == 1 and len(kwargs) == 1)
+    assert that(spy).was.NOT.called.with_arguments(50)
+    assert that(spy).was.NOT.called.with_exact_arguments(30)
+    assert that(spy).was.NOT.called.with_no_argument()
+    assert that(spy).was.called.with_exact_arguments(30, arg="string")
+    with pytest.raises(Assertion):
+        assert that(spy).was.called.with_arguments(50)
+    with pytest.raises(Assertion):
+        assert that(spy).was.called.with_exact_arguments(30)
+    with pytest.raises(Assertion):
+        assert that(spy).was.called.with_no_argument()
+
+def test_spy_seral_calls():
+    spy = Spy(lambda *args, **kw: None)
+    obj = object()
+    spy()
+    spy(30, arg="string")
+    spy(obj, 30, example=obj, none=None)
+    assert that(spy).was.called
+    assert that(spy).was.called.more_than.once
+    assert that(spy).was.called.more_than.twice
+    assert that(spy).was.NOT.called.more_than.thrice
+    assert that(spy).was.called.at_most.thrice
+    assert that(spy).was.called.at_least.thrice
+    assert that(spy).was.called.three.times
+    assert that(spy).was.called.less_than(4).times
+    # Check arguments
+    assert that(spy).was.called.once.with_no_argument()
+    assert that(spy).was.called.at_most.once.with_no_argument()
+    assert that(spy).was.called.twice.with_arguments(30)
+    assert that(spy).was.NOT.called.less_than.twice.with_arguments(30)
+    assert that(spy).was.called.once.with_arguments(obj)
+    assert that(spy).was.called.once.with_arguments(arg="string")
+    assert that(spy).was.called.once.with_arguments(30, obj)
+    assert that(spy).was.called.once.with_arguments(none=None)
+    assert that(spy).was.NOT.called.with_arguments(None)
+    assert that(spy).was.NOT.called.with_arguments(obj, 30, arg="string")
+    with pytest.raises(Assertion):
+        assert that(spy).was.called.with_arguments(obj, 30, arg="string")
+    # Checking with exact arguments
+    assert that(spy).was.called.once.with_exact_arguments(30, arg="string")
+    assert that(spy).was.called.once.with_exact_arguments(obj, 30, example=obj, none=None)
+    assert that(spy).was.NOT.called.with_exact_arguments(obj, 30, arg="string")
+    with pytest.raises(Assertion):
+        assert that(spy).was.called.with_exact_arguments(obj, 30, arg="string")
+    # Check arguments matching
+    assert that(spy).has.NOT.been.called.with_arguments_matching(lambda a, kw: len(a) + len(kw) == 3)
+    assert that(spy).was.called.once.with_arguments_matching(lambda a, kw: len(a) + len(kw) == 2)
+    assert that(spy).was.called.once.with_arguments_matching(lambda a, kw: len(a) + len(kw) == 4)
+    with pytest.raises(Assertion):
+        assert that(spy).was.called.with_arguments_matching(lambda a, kw: len(a) + len(kw) == 3)
+
+
+def test_wrongful_expressions():
+    spy = Spy()
+    with pytest.raises(RuntimeError):
+        assert that(3).is_.less_than("str")
+    with pytest.raises(RuntimeError):
+        assert that(3).does.NOT.NOT.equal(3)
+    with pytest.raises(RuntimeError):
+        assert that(3).is_.an.instance_of("non type")
+    with pytest.raises(RuntimeError):
+        assert that(spy).was.called.more_than.at_least.once
+    with pytest.raises(RuntimeError):
+        assert that(spy).was.called.more_than.at_most.once
+    with pytest.raises(RuntimeError):
+        assert that(spy).was.called.more_than.less_than.once
+    with pytest.raises(RuntimeError):
+        assert that(spy).was.called.more_than.more_than.once
\ No newline at end of file