def build(self, bldr):
stage_names = sets.Set()<EOL>for source in self._sources:<EOL><INDENT>source._build(bldr, stage_names)<EOL><DEDENT>for source in self._sources:<EOL><INDENT>if not source._all_built():<EOL><INDENT>raise RuntimeError("<STR_LIT>")<EOL><DEDENT><DEDENT>
Builds the topology and returns the builder