diff --git a/simplex/simplex_dictionary.py b/simplex/simplex_dictionary.py
index 9e870dc..8393879 100644
--- a/simplex/simplex_dictionary.py
+++ b/simplex/simplex_dictionary.py
@@ -2,10 +2,18 @@
 
 import typing
 
+from abc import abstractmethod, ABC
 from fractions import Fraction
 
 from .magic_dictionary import magic_dictionary
 
+
+class Comparable(ABC):
+    @abstractmethod
+    def __lt__(self, other: typing.Any) -> bool:
+        ...
+
+
 # Type that can be converted by convert_value:
 convertable_type = typing.Union[str, int, Fraction]
 
@@ -18,12 +26,13 @@ def convert_value(value: convertable_type) -> Fraction:
 
 
 # Type of variables in a simplex_dictionary:
-V = typing.TypeVar("V")
+V = typing.TypeVar("V", bound=Comparable)
 
 
 class simplex_dictionary(typing.Generic[V]):
 
-    """ Class representing a dictionary for the simplex algorithm. The class contains
+    """
+    Class representing a dictionary for the simplex algorithm. The class contains
     multiple members representing the elements of a dictionary.
 
     Members:
@@ -76,37 +85,49 @@ class simplex_dictionary(typing.Generic[V]):
         self.z = Fraction(0)
 
     def _check_basic(self, key: V) -> typing.Optional[str]:
-        """ Check if the given key is a basic variable, returning None if it is,
-        and a custom exception string if not. Suitable for magic_dictionary use. """
+        """
+        Check if the given key is a basic variable, returning None if it is,
+        and a custom exception string if not. Suitable for magic_dictionary use.
+        """
         if key not in self.B:
             return "{} is not a basic variable.".format(key)
         return None
 
     def _check_non_basic(self, key: V) -> typing.Optional[str]:
-        """ Check if the given key is a non-basic variable, returning None if it is,
-        and a custom exception string if not. Suitable for magic_dictionary use. """
+        """
+        Check if the given key is a non-basic variable, returning None if it is,
+        and a custom exception string if not. Suitable for magic_dictionary use.
+        """
         if key not in self.N:
             return "{} is not a non-basic variable.".format(key)
         return None
 
     @property
     def variables(self) -> typing.List[V]:
-        """ Returns: The list of variables in this dictionary. """
+        """
+        Returns: The list of variables in this dictionary.
+        """
         return sorted(self.B + self.N)
 
     @property
     def B(self) -> typing.List[V]:
-        """ Returns: The list of basic variables for this dictionary. """
+        """
+        Returns: The list of basic variables for this dictionary.
+        """
         return self.__B
 
     @property
     def N(self) -> typing.List[V]:
-        """ Returns: The list of non-basic variables for this dictionary. """
+        """
+        Returns: The list of non-basic variables for this dictionary.
+        """
         return self.__N
 
     @property
     def a(self) -> magic_dictionary[V, magic_dictionary[V, Fraction]]:
-        """ Returns: The a matrix of this dictionary. """
+        """
+        Returns: The a matrix of this dictionary.
+        """
         return self._a
 
     @a.setter
@@ -171,7 +192,8 @@ class simplex_dictionary(typing.Generic[V]):
         self._z = convert_value(value)
 
     def name_latex(self, name: typing.Any) -> str:
-        """ Convert the given variable name to a clean latex name.
+        """
+        Convert the given variable name to a clean latex name.
 
         Args:
             name: The name of the variable to convert.
@@ -191,7 +213,8 @@ class simplex_dictionary(typing.Generic[V]):
         return s[0] + "_{" + s[1] + "}"
 
     def value_latex(self, value: Fraction) -> str:
-        """ Convert the given fraction to a latex fraction.
+        """
+        Convert the given fraction to a latex fraction.
 
         Args:
             value: The fraction to convert.
@@ -207,7 +230,8 @@ class simplex_dictionary(typing.Generic[V]):
         )
 
     def display(self, name: str = None):
-        """ Display this simplex dictionary on the standard Jupyter output.
+        """
+        Display this simplex dictionary on the standard Jupyter output.
 
         Args:
             name: Name of the dictionary.
diff --git a/tests/test_magic_dictionary.py b/tests/test_magic_dictionary.py
index 59bc462..6671867 100644
--- a/tests/test_magic_dictionary.py
+++ b/tests/test_magic_dictionary.py
@@ -4,8 +4,8 @@ from simplex.magic_dictionary import magic_dictionary
 
 
 def test_basic():
-    """ Tests that a non-customized magic_dictionary acts
-    as a standard python dictionary. """
+    """Tests that a non-customized magic_dictionary acts
+    as a standard python dictionary."""
 
     d = magic_dictionary()
     assert len(d) == 0