> From: Pierpaolo Bernardi <bernardp@cli.di.unipi.it>
> 
> Usually, that is, in the most straightforward implementation, you rebuild
> the path on the way up.  By throwing the exception, you don't cons any new
> node.
> [...]
> For this to work, you should either have a low-level pointer equality
> operator (present in OCaml, but not in other func. languages), or you
> must return a flag to signal whether the returned tree is unchanged.
> Both variants are ugly and cumbersome, IMO.

This theme was discussed last year under a thread called
 "List.filter in Ocaml 2.02"

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