code: def add_sources(self, sources):
          
          # TODO(batch):  extend to multiple batches
          assert (len(self.batches) == 1)
          # TODO(batch):  remove duplicates?
          sources = [x for x in sources if x not in self.batches[0].sources]
          self.batches[0].sources.extend(sources)
docs: Add source files to this Executor's list.  This is necessary
          for "multi" Builders that can be called repeatedly to build up
          a source file list for a given target.