type ex =
  | E : {
      module_ :
        (module S.Analyzer
           with type Expression.t = 'a
            and type Expression.t' = 'b
            and type Instruction.t = 'c
            and type Instruction.t' = 'd
            and type Location.t = 'e
            and type context = 'f);
      expr_witness : 'a Type.Id.t;
      expr' : 'b Type.Id.t;
      instr_witness : 'c Type.Id.t;
      instr' : 'd Type.Id.t;
      location_witness : 'e Type.Id.t;
      context : 'f Type.Id.t;
    }
      -> ex  (** Type of check to apply *)

let build :
    ?location_id:'a Type.Id.t ->
    ?context_id:'b Type.Id.t ->
    (module S.Analyzer
       with type Expression.t = _
        and type Expression.t' = _
        and type Instruction.t = _
        and type Instruction.t' = _
        and type Location.t = 'a
        and type context = 'b) ->
    ex =
 fun ?location_id ?context_id module_ ->
  let expr_witness = Type.Id.make ()
  and expr' = Type.Id.make ()
  and instr_witness = Type.Id.make ()
  and instr' = Type.Id.make ()
  and location_witness =
    match location_id with Some v -> v | None -> Type.Id.make ()
  and context = match context_id with Some v -> v | None -> Type.Id.make () in
  E
    {
      module_;
      expr_witness;
      expr';
      instr_witness;
      instr';
      location_witness;
      context;
    }