def add_edge(self, x, y, label=None):
self.adjacency_list[x].append((y, label))<EOL>if x not in self.reverse_list[y]:<EOL><INDENT>self.reverse_list[y].append(x)<EOL><DEDENT>
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``