The challenge is to decide on the specification of what your function inputs and what it outputs. Get specification right, implement it as you describe (ignorin

The challenge is to decide on the specification of what your function inputs and what it outputs. Get specification right, implement it as you describe (ignoring the task of your recursion goblins), and it'll work. Get the specification wrong, and the implementation will either be difficult or impossible. It is helpful, of course, if someone else has defined a workable specification for you already.