docs: def variables(self):
          """
          Returns variables in the logical network
  
          Returns
          -------
          set[str]
              Unique variables names
          """
          variables = set()
          for v in self.nodes_iter():
              if isinstance(v, Clause):
                  for l in v:
                      variables.add(l.variable)
              else:
                  variables.add(v)
          return variables