compound context seq fn: op-union

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

    examples:
        -- learn some toy knowledge:
        days |weekday> => split[" "] |Monday Tuesday Wednesday Thursday Friday>
        days |weekend> => split[" "] |Saturday Sunday>

        -- now find their union:
        op-union[days] split[" "] |weekday weekend>
            |Monday> + |Tuesday> + |Wednesday> + |Thursday> + |Friday> + |Saturday> + |Sunday>

    see also:
        common, op-sum