Jacques Garrigue wrote:
> From: Alessandro Baretta <a.baretta@studio.baretta.com>

> Looks like your implementation contains at least `Options, while the
> interface doesn't. Since it is in a contravariant position, you should
> be able to remove it from the implementation by subtyping.
> 
> Jacques Garrigue

Ah, yes! The error message was so long that I missed the relevant spot, and the 
`&' sign tricked me into thinking that the issue was there. And, yes, the 
problem is the "> `Options" constraint, which appears in the inferred signature 
of the function, but not in the mli.

Thank you very much.

Alex