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? $$ f: x \mapsto x^{x^{x^{x^x}}} $$