Extract Expr
I am a big fan of Eval(Substitute(Expr(...), Expr(..), Expr(.))): ) The only disadvantage: the Expr(...), Expr(..), Expr(.)... which brought me to the idea of subst() - substitute() 2.0 ... a function which doesn't evaluate it's arguments.
While playing around with Expression Handling, I noticed that the desired logic for Subst() is quite close to the one of Extract Expr(): no need to use Expr(), ...