anchor.diff:    ;
   
   element :
  -     expr_in_array
  +     expr
     ;
   
   //non-empty
  
positive.code:              isPrefixOperator(prevToken) || isPostfixOperator(nextToken) ||
               isLParen(prevToken) || isRParen(nextToken));
  }
  }
  
  @lexer::header {
  
negative.code: prefix_arg :
       pow_arg
    |  t_NOT t_FILLER? prefix_arg
    |  (t_PLUS | t_MINUS) t_FILLER? prefix_arg //NB: plus_arg may break if this is written as two rules
    ;
  
  pow_arg :
  