start at a, (then -1 = a[-1] which doesn't exist), and go in reverse order until the number is less than 9, which zero is. You want, a[end:start-1:-1], but why even try to reinvent the wheel that is already working.
With this knowledge the rules are simple.
start and stop values will always be the same for posetive step values(default is step=1)
start and stop values will always be the same for negative step values.