built in compound operators: select

select:
    description:
        select[k1, k2] sp
        select the k1'th to the k2'th elements from the given superposition
        indices start from 1, not 0
        also, final element has index -1, 2nd last element -2, and so on.

    examples:
        select[2, 4] split |abcdefg>
            |b> + |c> + |d>

        select[-3, -1] split |abcdefg>
            |e> + |f> + |g>

    see also:
        sselect