anchor.diff:  
   class_method_def
     : method_mod_list KW_FUNCTION is_ref name=ID
  -      LPAREN formal_parameter_list RPAREN method_body
  +      (LPAREN formal_parameter_list RPAREN)? method_body
         -> ^(KW_FUNCTION $name method_mod_list is_ref
  -                        formal_parameter_list method_body)
  +                        formal_parameter_list? method_body)
     ;
   
   method_mod_list
  
positive.code:   ;
  
  function_def
    : KW_FUNCTION name=ID LPAREN formal_parameter_list RPAREN
      LBRACE statement* RBRACE -> ^(KW_FUNCTION $name
        formal_parameter_list statement*)
    ;
  
  formal_parameter_list
    : formal_parameter (COMMA formal_parameter_list)?
        -> formal_parameter formal_parameter_list?
    ;
  
  formal_parameter
  
negative.code:   ;
  
  indexed_function_call
    : name=ID (LPAREN actual_parameter_list RPAREN)? call_result_index* ->
      ^(Call $name actual_parameter_list? call_result_index*) ;
  
  actual_parameter_list
    : actual_parameter (COMMA actual_parameter)*
      -> actual_parameter+
    ;
  
  actual_parameter
  