Nat fixes

demos/contractBasics.tri

@@ -8,8 +8,8 @@ myAndC = (c1 c2 value rest :
     (c1 value rest))
 
 -- Plain predicates lifted into contracts with a diagnostic message.
-natural? = guardC "natural" (n : gte? n 0)
-nonZero? = guardC "non-zero" (n : not? (isZero? n))
+natural? = guardC "natural" isNat?
+nonZero? = guardC "non-zero" (n : and? (isNat? n) (not? (isZero_? n)))
 
 -- Phantom annotations let point-free definitions carry their own contracts.
 -- The base library now uses the same syntax, so head/tail/div etc. are