sequence function: sread-range

    description:
        sread-range(start-idx, finish-idx) input-seq
        the sequence read-range function
        acts just like sselect[a,b] except a and b are now kets, not constants.
        is also almost the same as: sread(range(start-idx, finish-idx)) input-seq
        but there are some differences. eg, sread-range can handle negative indices too.
        index values start at 1, not 0. So 1 is the first element, 2 is the 2nd element, and so on.
        negative index values work too. eg, -1 is the last element in the sequence, -2 is the 2nd last, and so on.
        if the index value is > length of input-seq, then those index values are ignored

    examples:
        -- standard usage:
        sread-range(|2>, |5>) ssplit |abcdefgh>
            |b> . |c> . |d> . |e>
        
        -- with negative index value:
        sread-range(|2>, |-3>) ssplit |abcdefgh>
            |b> . |c> . |d> . |e> . |f>

    see also:
        sread, swrite, swrite-range, sselect