'Unmark an edge (or all edges)'

def unmark_edge(self, edge=None):

    if (edge is None):
        old_marked_edges = list(self._marks.keys())
        self._marks = {}
        for edge in old_marked_edges:
            self._color_edge(edge)
    else:
        del self._marks[edge]
        self._color_edge(edge)
