Possibly a difficult grammar is the grammar of OCaml value definition...
structure_item:
    LET rec_flag let_bindings
;
rec_flag:
    /* empty */
  | REC
;
val_ident:

    LIDENT
  | LPAREN operator RPAREN
;
let_bindings:
    let_binding
  | let_bindings AND let_binding
;
let_binding:
    val_ident fun_binding
  | pattern EQUAL seq_expr
;
fun_binding:

    strict_binding
  | type_constraint EQUAL seq_expr
;
strict_binding:
    EQUAL seq_expr
  | labeled_simple_pattern fun_binding
;

Indeed there are some "unknown" items, but the difficult part here is trying to parse the function declaration with 

variable number of parameters, using no (slow) recursion. 

Hm... maybe, I don't know, I found this both hard to think of and hard to understand. 

- Tom