From: Claudio Sacerdoti Coen <sacerdot@cs.unibo.it>

>  Obviously, I was a bit too quick! I think that the trick works only up to
>  the point where the mutual recursion is fake. This is a slightly more
>  complex example:
> 
> # type -'a t;;
> 
> # let f (x : [> `A] t) = (x : [`A] t);;
> val f : [ `A] t -> [ `A] t = <fun>

If you just put two annotations, they will be unified.
You need an explicit coercion to get the type you expect:

# let f x = (x : [> `A] t :> [`A] t);;
val f : [> `A] t -> [ `A] t = <fun>

Jacques Garrigue
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