an alternate constructor
    is provided:

    >>> r = RangeMap.left({1: 'a', 4: 'b', 7: RangeMap.undefined_value})
    >>> r[1], r[2], r[3], r[4], r[5], r[6]
    ('a', 'a', 'a', 'b', 'b', 'b')

    c