code: def children_after_parents(self, piper1, piper2):
          
          if piper1 in self[piper2].deep_nodes():
              return 1
          elif piper2 in self[piper1].deep_nodes():
              return - 1
          else:
              return 0
docs: Custom compare function. Returns ``1`` if the first ``Piper`` instance 
          is upstream of the second ``Piper`` instance, ``-1`` if the first 
          ``Piper`` is downstream of the second ``Piper`` and ``0`` if the two 
          ``Pipers`` are independent.
          
          Arguments:
          
            - piper1(``Piper``) ``Piper`` instance.
            - piper2(``Piper``) ``Piper`` instance.