def ranges(iterable):
    iterable = iter(iterable)
    start = stop = iterable.next()
    for cur in iterable:
        stop += 1
        if cur != stop:
            yield yield_range(start, stop, '-')
            start = stop = cur
    yield yield_range(start, stop, '-')

def yield_range(start, stop, delimiter):
    if start == stop:
        return str(stop)
    else:
        return "%d%s%d" % (start, delimiter, stop)

if __name__ == '__main__':
    print list(ranges([1, 2, 3, 5, 6, 11]))
    print list(ranges([1, 3, 4, 12, 13]))
    print list(ranges([1, 2, 3, 4, 7, 8]))