function 2: srange

srange:
    description:
        srange(start, stop)
        srange(start, stop, step)
        returns a sequence of kets starting at start, and finishing at stop
        with step size step, if it is provided, otherwise step with size 1
        if start and stop have categories, then so does the result
        if the categories differ, then return the empty ket |>
        NB: unlike range, there is no (currently?) infix operator version

    examples:
        srange(|1>, |6>)
            |1> . |2> . |3> . |4> . |5> . |6>

        srange(|number: 7>, |number: 13>)
            |number: 7> . |number: 8> . |number: 9> . |number: 10> . |number: 11> . |number: 12> . |number: 13>

        srange(|2>, |4>, |0.5>)
            |2> . |2.5> . |3> . |3.5> . |4>

        srange(|8>, |3>, |-1>)
            |8> . |7> . |6> . |5> . |4> . |3>

    see also:
        range, seq2sp