You can return whatever you like, but this way you are shifting the burden of tree iteration onto the consumer of your output. It also seems curious to state how you "avoid any sort of reverse or append of the lists" when you produce a tree.
Ah, well I guess I was trying to return a value for a human rather than another function, my naive implementation used a reverse rather than a curried function, and append was mentioned in the thread by a reply which I misinterpreted as attempting to return the same thing I was.
It shows that you are making 27 leaf calls to produce a collection of 27 items, so there's nothing to memoize since the lambdas contain information essential to the return value.
Yes I undestand that, but I'm even more confused than before because the (timed) performance is equivilent to the memoized version despite this.