Let X,Y be random variables defined on a probability space $(\Omega,S,P)$ and let $\phi(X,Y)$ be a borel function of X and Y.
I want to prove the statement $$E\{ \phi(X,Y)|Y=y \} = E\{\phi(X,y)|Y=y\} $$
It seems intuitively easy, but I can't prove it rigorously in the continuous case (if X, Y are continuous) .