function 2: range

range:
    description:
        range(start, stop)
        range(start, stop, step)
        returns a superposition 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 |>
        now partially implemented as: start .. stop
        in this form, you can't specify the step size

        Now we can handle 2D ranges too:
        Eg: |-1: -1> .. |1: 1>

    examples:
        range(|1>, |6>)
            |1> + |2> + |3> + |4> + |5> + |6>

        range(|number: 7>, |number: 13>)
            |number: 7> + |number: 8> + |number: 9> + |number: 10> + |number: 11> + |number: 12> + |number: 13>

        range(|2>, |4>, |0.5>)
            |2> + |2.5> + |3> + |3.5> + |4>

        range(|8>, |3>, |-1>)
            |8> + |7> + |6> + |5> + |4> + |3>

        -- an example using the new infix operator:
        |13> .. |17>
            |13> + |14> + |15> + |16> + |17>

        -- an example of 2D range using the new infix operator:
        |0: 0> .. |1: 1>
            |0: 0> + |0: 1> + |1: 0> + |1: 1>

        -- and another:
        |-1: -1> .. |1: 1>
            |-1: -1> + |-1: 0> + |-1: 1> + |0: -1> + |0: 0> + |0: 1> + |1: -1> + |1: 0> + |1: 1>

    see also:
        srange, sp2seq