docs: def _raise_on_bad_representation(representation):
          """Ensure that representation is a known Stanford Dependency
          representation (raises a ValueError if the representation is
          invalid)."""
          if representation not in REPRESENTATIONS:
              repr_desc = ', '.join(map(repr, REPRESENTATIONS))
              raise ValueError("Unknown representation: %r (should be one "
                               "of %s)" % (representation, repr_desc))