> I think your solution is to fully use the "Objective" part of OCaml, 
> that is, using subtyping (technically it's polymorphic rows typing) :

Hi,

Yes, you're right.  Doing away with the inheritance -- while having the
disadvantage of forcing the redeclaration of all common fields -- does
allow me to throw away the ugliness of the option types.  It is in the
end the lesser of two evils...

Cheers,
Dario



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