invariant-inference.mo:5.11-5.32: type error [M0098], cannot implicitly instantiate function of type
  <T>(Nat, T) -> [var T]
to argument of type
  (Nat, Nat)
because implicit instantiation of type parameter T is under-constrained with
  Nat  <:  T  <:  Any
where
  Nat  =/=  Any
so that explicit type instantiation is required
invariant-inference.mo:15.11-15.13: type error [M0050], literal of type
  Int
does not have expected type
  Nat