code: def _ExpandArtifactFilesSource(self, source, requested):
      
      expanded_source = rdf_artifacts.ExpandedSource(base_source=source)
      sub_sources = []
      artifact_list = []
      if "artifact_list" in source.attributes:
        artifact_list = source.attributes["artifact_list"]
      for artifact_name in artifact_list:
        if artifact_name in self.processed_artifacts:
          continue
        artifact_obj = artifact_registry.REGISTRY.GetArtifact(artifact_name)
        for expanded_artifact in self.Expand(artifact_obj, requested):
          sub_sources.extend(expanded_artifact.sources)
      expanded_source.artifact_sources = sub_sources
      expanded_source.path_type = self._path_type
      return [expanded_source]
docs: Recursively expands an artifact files source.