def split_gap(opt_model, idx, lcl_pt):
    """ split g=gap[idx] into t_old = t_0 + t_k using t_0 = lcl_pt.x """
    g = opt_model.seq_model.gaps[idx]
    x, y = lcl_pt
    t_k = g.thi - x
    t_0 = x
    return g, t_0, t_k