\def
\lambda f:int \to int. \lambda x : pos .f (nat2int (pos2nat x)).
+(* This used to test eq_f as a coercion. However, posing both eq_f and sym_eq
+ as coercions made the qed time of some TPTP problems reach infty.
+ Thus eq_f is no longer a coercion (nor is sym_eq).
theorem coercion_svelta : \forall T,S:Type.\forall f:T \to S.\forall x,y:T.x=y \to f y = f x.
intros.
apply ((\lambda h:f y = f x.h) H).
qed.
+*)
variant pos2nat' : ? \def pos2nat.