compound sigmoids: not-threshold-filter

not-threshold-filter:
    description:
        not-threshold-filter[t] ket
        if x <= t, return x, else return 0

    examples:
        not-threshold-filter[2] (3|a> + 2.2|b> - 3 |c> + |d>)
            0|a> + 0|b> - 3|c> + |d>

    see also:
        threshold-filter