code: def try_add_variable(self, variable_name: str, replacement: VariableReplacement) -> None:
          
          if variable_name not in self:
              self[variable_name] = replacement.copy() if isinstance(replacement, Multiset) else replacement
          else:
              existing_value = self[variable_name]
  
              if isinstance(existing_value, tuple):
                  if isinstance(replacement, Multiset):
                      if Multiset(existing_value) != replacement:
                          raise ValueError
                  elif replacement != existing_value:
                      raise ValueError
              elif isinstance(existing_value, Multiset):
                  if not isinstance(replacement, (tuple, list, Multiset)):
                      raise ValueError
                  compare_value = Multiset(replacement)
                  if existing_value == compare_value:
                      if not isinstance(replacement, Multiset):
                          self[variable_name] = replacement
                  else:
                      raise ValueError
              elif replacement != existing_value:
                  raise ValueError
docs: Try to add the variable with its replacement to the substitution.
  
          This considers an existing replacement and will only succeed if the new replacement
          can be merged with the old replacement. Merging can occur if either the two replacements
          are equivalent. Replacements can also be merged if the old replacement for the variable_name was
          unordered (i.e. a :class:`~.Multiset`) and the new one is an equivalent ordered version of it:
  
          >>> subst = Substitution({'x': Multiset(['a', 'b'])})
          >>> subst.try_add_variable('x', ('a', 'b'))
          >>> print(subst)
          {x ↦ (a, b)}
  
          Args:
              variable:
                  The name of the variable to add.
              replacement:
                  The replacement for the variable.
  
          Raises:
              ValueError:
                  if the variable cannot be merged because it conflicts with the existing
                  substitution for the variable_name.