What is the Psi(x) variable binding operator? In the Free Variable article on Wikipedia, it lists these:

as variable-binding operators. I have seen all of them during my math studies, except for the psi operator. What does $\psi x$ mean in this context?

My first impulse in seeing ‘$\varphi x$’ is to read it as a variation on the logical schema (‘$\varphi(x)$’, ‘$F x$’, etc.), which, if that was the intented meaning, should, I believe, preclude it from a list of variable-binding operators. I would vote up for removal from the article if I could.
