4. Assume that `Ψ` is some fixed point combinator; we're not telling you which one. (You can just write `Psi` in your homework if you don't know how to generate the symbol `Ψ`.) Prove that `Ψ Ψ` is a fixed point of itself, that is, that `Ψ Ψ <~~> Ψ Ψ (Ψ Ψ)`.
-<!-- Proof: YY --> Y(YY) --> YY(Y(YY)); YY(YY) --> YY(Y(YY)) -->
+<!-- Proof: YY ~~> Y(YY) ~~> YY(Y(YY)); YY(YY) ~~> YY(Y(YY)) -->
## Writing recursive functions ##