'Add an edge from distribution *x* to distribution *y* with the given
*label*.
:type x: :class:`distutils2.database.InstalledDistribution` or
:class:`distutils2.database.EggInfoDistribution`
:type y: :class:`distutils2.database.InstalledDistribution` or
:class:`distutils2.database.EggInfoDistribution`
:type label: ``str`` or ``None``'

def add_edge(self, x, y, label=None):

    self.adjacency_list[x].append((y, label))
    if (x not in self.reverse_list[y]):
        self.reverse_list[y].append(x)
