'Add a new edge to the chart, using a pointer to the previous edge.'

def insert_with_backpointer(self, new_edge, previous_edge, child_edge):

    cpls = self.child_pointer_lists(previous_edge)
    new_cpls = [(cpl + (child_edge,)) for cpl in cpls]
    return self.insert(new_edge, *new_cpls)
