So if I understand correctly, these are examples of free variables: (all occurrences of $x$ are free) $ x*0 $ $ 0+x*0 $ $ f: y \mapsto x*0 $ $ x*12345*(1-1) $ $ x*12345*(5-(10/2)) $
What is an example of a variable that is not free (expensive variable?)
Is this $x$ here one? $ f: x \mapsto x^{x^{x^{x^x}}} $