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

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)