Dear Jacques:
    I was reading the manual of chap4, it says
*tag-spec-full*::=`*tag-name<http://www.askra.de/software/ocaml-doc/4.00/manual011.html#tag-name>
*  [ of *typexpr<http://www.askra.de/software/ocaml-doc/4.00/types.html#typexpr>
* ]  { & *typexpr<http://www.askra.de/software/ocaml-doc/4.00/types.html#typexpr>
* }   ∣ *typexpr<http://www.askra.de/software/ocaml-doc/4.00/types.html#typexpr>
*

    I don't follow the (&) operation here, it means intersection types ?

    I managed to make a working example:


          type 'a ab = [< `a|`b] as 'a ;;
          type 'a ac = 'a constraint 'a = [< `a | `c ]
          type ('a,'b) m = [< `m of 'a ab & 'a ac ] as 'b;;
          let u : ('a,'b) m = `m `a;;
          (* val u : ([ `a ], [ `m of [ `a ] ab ]) m = `m `a *)

   But still don't understand its use case, can you explain a bit about
this?

Many thanks
-- 
Regards
-- Bob