type token =
  A | B | C of int | EOF
;;

%%

%token A
%token B
%token <> C
%token EOF

%%

r():
  one:s()
  {{ }}
  b:B
  two:B?
  three:s()
  {{ prerr_endline ("Result: " ^ string_of_int three) }}
? {{ prerr_endline ("ERROR: " ^ !yy_position) }}

s():
  A 
  {{ }}
  {{ prerr_endline "A"; 0 }}
| B 
  {{ }}
  {{ prerr_endline "B"; 0 }}
| n:C
  {{ }}
  {{ prerr_endline ("C: " ^ string_of_int n); n }}
%%

let input = ref [ A; B; B; B; C 5; EOF ] in
let current() = List.hd !input in
let next_token () =
  prerr_endline "get_next";
  input := List.tl !input;
  List.hd !input
in
parse_r current next_token
;;