code: def _is_reference(bpe):
      
      if isinstance(bpe, _bp('ProteinReference')) or \
          isinstance(bpe, _bpimpl('ProteinReference')) or \
          isinstance(bpe, _bp('SmallMoleculeReference')) or \
          isinstance(bpe, _bpimpl('SmallMoleculeReference')) or \
          isinstance(bpe, _bp('RnaReference')) or \
          isinstance(bpe, _bpimpl('RnaReference')) or \
          isinstance(bpe, _bp('EntityReference')) or \
          isinstance(bpe, _bpimpl('EntityReference')):
          return True
      else:
          return False
docs: Return True if the element is an entity reference.