docs: def _reverse_to_source(self, target, group1):
          """
          Args:
              target (dict): A table containing the reverse transitions for each state
              group1 (list): A group of states
          Return:
              Set: A set of states for which there is a transition with the states of the group
          """
          new_group = []
          for dst in group1:
              new_group += target[dst]
          return set(new_group)