Returns ``Piper`` instances, which are inputs to the pipeline i.e. have 
        no incoming pipes (outgoing dependency edges).

def get_inputs(self):
        """
        Returns ``Piper`` instances, which are inputs to the pipeline i.e. have 
        no incoming pipes (outgoing dependency edges). 
        
        """
        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