def _is_small_molecule(pe):
    """Return True if the element is a small molecule"""
    val = isinstance(pe, _bp('SmallMolecule')) or \
            isinstance(pe, _bpimpl('SmallMolecule')) or \
            isinstance(pe, _bp('SmallMoleculeReference')) or \
            isinstance(pe, _bpimpl('SmallMoleculeReference'))
    return val