docs: def formulas_iter(self):
          """
          Iterates over all variable-clauses in the logical network
  
          Yields
          ------
          tuple[str,frozenset[caspo.core.clause.Clause]]
              The next tuple of the form (variable, set of clauses) in the logical network.
          """
          for var in it.ifilter(self.has_node, self.variables()):
              yield var, frozenset(self.predecessors(var))