code: def add_edge(self, x, y, label=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)
docs: 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``