It's clear, minimalist, insightful, and even has an in-page live code demo. Bonus points for including fixed-points as a key concept, and memoization as an extremely useful technique.
That being said, I'm a big fan of decorators, so I expressed the memoization using a decorator, rather than baking it into Ymem.
On the other hand, trampolining is the perfect application for a special-purpose Y combinator, and thus the decoupled Trampoline.
It's clear, minimalist, insightful, and even has an in-page live code demo. Bonus points for including fixed-points as a key concept, and memoization as an extremely useful technique.