anchor.diff:      | proc_type
       | path
       | '_'
  -] { printf("ty\n"); }
       ;
   
   /// 8.1.3 Tuple types - handled in ty
  
positive.code:                  | item_impl
                   | item_struct
                   ]
          { printf("item_or_view_item\n"); }
      ;
  
  /// 6.1.1 Type Parameters
  
negative.code: /// 7.2.9 Index expressions - handled by dot_or_call_expr
  /// 7.2.10 Unary operator expressions
  prefix_expr
      : '!' prefix_expr
      | '-' prefix_expr
      | '*' prefix_expr
      | '&' /* LIFETIME? */ MUT? prefix_expr // ambiguity: lifetime after & may be a loop label
      | '~' prefix_expr
      | BOX prefix_expr // ambiguity: what to do with box ( expr? )?
      | dot_or_call_expr
      ;
  
  /// 7.2.11 binary operator expressions
  