If we look at the following piece of code:

===>
type dof_region_spec = (string * bool * (int option array)) array;;
type opt_field_restriction = dof_region_spec option;;

let make_field ?(name="Field") ?restriction ?constant_value mwe =
   let restriction:opt_field_restriction = restriction in
	failwith "FOO";;
<===

...then make_field types as:

===>
val make_field :
   ?name:string -> ?restriction:<hidden> -> ?constant_value:'a -> 'b -> 'c
<===

The <hidden> actually should not show up here. What is going on?

-- 
best regards,
Thomas Fischbacher
tf@functionality.de