Attach an invariant to a type

["Dawid Toton" <d0@wp.pl>]

What should I do if I have need for the following? Does already exist 
any equivalent solution?

I'd like to write:

type subindex = int invariant x -> (x>=10)&&(x<=100)

let doit (a:subindex) (b:subindex) =
  let result = some_operation a b in
  (result:subindex)

And it should be translated to:

type subindex = int
let subindex_invariant x = (x>=10)&&(x<=100)

let doit (a:subindex) (b:subindex) =
  assert (subindex_invariant a);
  assert (subindex_invariant b);
  let result = some_operation a b in
  assert (subindex_invariant result);
  (result:subindex)

Am I going right direction at all?

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