built in operators: do-you-know

do-you-know:
    description:
        do-you-know seq
        returns |no> if seq is the empty ket |>
        returns |yes> otherwise
        frequently useful with if, op-if() and value-if()

    examples:
        do-you-know |>
            |no>

        do-you-know |...>
            |yes>

    see also:
        op-if, value-if