A13: dom g = X by FUNCT_2:def 1;
A14: dom (eq (f,g)) = (dom f) /\ (dom g) by Def7;
A15: dom f = X by FUNCT_2:def 1;
rng (eq (f,g)) c= INT ;
hence eq (f,g) is Function of X,INT by A14, A15, A13, FUNCT_2:2; :: thesis: verum