Section function_evaluation_proof.
Variable F X Y : Ens.
Hypothesis f : is_a_function F.
Hypothesis i : IN (PAIR X Y) F.
Let Z := EV F X.
Let step1 : IN X (Domain F).
Proof Domain_th1 F X Y i.
Let step2 : IN (PAIR X Z) F.
Proof EV_th1 F X step1.

Lemma fe_Out : EQ Y Z.