def bond_idx_to_symb(idxs, idx_symb_dct):
    """ Convert a list of bond idxs ((a1, b1), (a2, b2), ..., (an, bn))
        to pairs of atom symbols
    """
    return tuple(
        (idx_symb_dct[idx1], idx_symb_dct[idx2]) for (idx1, idx2) in idxs
    )