def _get_partner(pdb_filename):
    """
    Get binding mode of pdb file.  x if none.

    e.g. 11as_r_u.pdb would give u
    """
    if '_r' in pdb_filename or '_2' in pdb_filename:
        return 'r'
    elif '_l' in pdb_filename or '_1' in pdb_filename:
        return 'l'
    else:
        return 'x'
