def _neighbor_keys_from_bond_keys(key, bnd_keys):
    """ Determine neighbor keys of an atom from the bond keys
    """
    nkeys = []
    for bnd_key in bnd_keys:
        if key in bnd_key:
            nkey, = bnd_key - {key}
            nkeys.append(nkey)
    return frozenset(nkeys)