def start(self):
<EOL>pipers = self.postorder()<EOL>for piper in pipers:<EOL><INDENT>piper.start(stages=(<NUM_LIT:0>, <NUM_LIT:1>))<EOL><DEDENT>for piper in pipers:<EOL><INDENT>piper.start(stages=(<NUM_LIT:2>,))<EOL><DEDENT>
Given the pipeline topology starts ``Pipers`` in the order input -> 
output. See ``Piper.start``. ``Pipers`` instances are started in two 
stages, which allows them to share ``NuMaps``.