anchor.diff:      ;
   
   variable
  -    : DOLLAR identifier
  +    : CARAT? DOLLAR? identifier
       ;
   
   set
  
positive.code:     ;
  
  param
      : identifier
      ;
  
  routinebody
  
negative.code:     ;
  
  term
      : variable
      | NUMBER
      | LPAREN expression RPAREN
      ;
  
  