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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_sources(self, sources):
 """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."""

        # 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)