I've got a type error:

This type of expression, <height:int; width:int; _..> -> unit, contains type 
variables that cannot be generalized

which appears with the form:

let f =
  let t = ref None in
  fun data -> ...

but not in the unnested form:

let t = ref None

let f =
  fun data -> ...

Is that supposed to happen? I thought those two forms were exactly equivalent 
(except for polluting the outer namespace in the latter case).

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
Objective CAML for Scientists
http://www.ffconsultancy.com/products/ocaml_for_scientists