> 1) is there any reason (unclear semantics, scope-holing, etc.) why the O'Caml
> module system won't allow an "include <signature>" feature?

Actually, "include <signature>" is available in O'Caml as it was in
Caml Special Light. It was a last-minute addition that never got
documented, but since it causes no harm and seems to be useful, I'll
keep it and document it for the next release.

> b) My second question concerns module coercion by means of module signatures.
> Sharing constraints between module types is really necessary for code
> abstraction, but I found "value aliasing" really useful as well (I don't
> know the exact term for the feature so I will describe it).

The only coercions that can be performed with a signature constraint
(str : sig) are restrictions, i.e. hide components or make their type
info less precise. I doubt renaming of structure components has its
place there. But it's easy (and just as efficient) to do the coercion
yourself, e.g.:

  module Node = struct
    type node 
    let less_depth = ...
    let less_dist = ...
    ...
  end

  module DepthFirstPQueue =
    MakePQueue (struct type t = Node.node let leq = Node.less_depth end)

  module BestFirstPQueue =
    MakePQueue (struct type t = Node.node let leq = Node.less_dist end)

Regards,

- Xavier Leroy