compound context seq fn: op-sum

op-sum:
    description:
        op-sum[op] seq
        find the sum of op applied to each of the kets in seq
        while trying to preserve sequence structure

    examples:
        seq |one> => ssplit |abcde>
        seq |two> => ssplit |uvw>
        seq |three> => ssplit |xyz>

        op-sum[seq] split[" "] |one two three>
            |a> + |u> + |x> . |b> + |v> + |y> . |c> + |w> + |z> . |d> . |e>

    see also:
        common, op-union