code: def get_inputs(self):
          
          start_p = [p for p in self.postorder() if not self.outgoing_edges(p)]
          self.log.debug('%s got input pipers %s' % (repr(self), start_p))
          return start_p
docs: Returns ``Piper`` instances, which are inputs to the pipeline i.e. have 
          no incoming pipes (outgoing dependency edges).