Indeed: it looks like on each iteration of `execute()`, exactly one leaf node is popped from `left`, no more and no less. So since there's nothing that can add more nodes to `left` after the program is first loaded, there's no way to call `execute()` more times than the total length of the program.

Another fix I believe sufficient (and perhaps a bit more interesting) would be an operation to exchange `left` and `right`, so that the program is replaced with the data, and vice versa.