built in compound operators: drop-below

drop-below:
    description:
        drop-below[t] sp
        drop all kets with coeff < t from the given superposition

    examples:
        drop-below[2] (-1 |a> + 0|b> + |c> + 2|d> + 3|e> + 4|f>)
            2|d> + 3|e> + 4|f>

    see also:
        drop, drop-above, sdrop