Programming in Lua

Write a coroutine backSubs. This takes a single parameter s, which must be a string. It yields all substrings of the reverse of s: first the unique length-zero substring, then all length-one substrings, and so on, ending with the reverse of s itself.

For example, if backSubs("abca") is called (as a coroutine), then it should yield the following, in order:

"" "a" "c" "b" "a" "ac" "cb" "ba" "acb" "cba" "acba"

I need some help with it, thanks for any help I can get!