Unroll [a recursive function]
as an explicit call tree for
a small input: every call, its
arguments, its return, in
order. Then the iterative
version.

See a Recursion Unrolled

The call tree drawn out, then the iterative twin.

Prompt

Show me [a recursive function] unrolled as an explicit call tree for a small input — every call, its arguments, what it returns, and in what order. Then show the same thing written iteratively.