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):
        """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``
        """
        self.adjacency_list[x].append((y, label))
        # multiple edges are allowed, so be careful
        if x not in self.reverse_list[y]:
            self.reverse_list[y].append(x)