docs: def add_variable(self, variable, card=0):
          """
          Add a variable to the model.
  
          Parameters:
          -----------
          variable: any hashable python object
  
          card: int
              Representing the cardinality of the variable to be added.
  
          Examples:
          ---------
          >>> from pgmpy.models import MarkovChain as MC
          >>> model = MC()
          >>> model.add_variable('x', 4)
          """
          if variable not in self.variables:
              self.variables.append(variable)
          else:
              warn('Variable {var} already exists.'.format(var=variable))
          self.cardinalities[variable] = card
          self.transition_models[variable] = {}