anchor.diff:  
   
   REGISTER
  -   : A X | B C | C X | D X | C I | D I | B P | S P | I P | C S | D S | E S | S S
  +   : A X | B X | C X | D X | C I | D I | B P | S P | I P | C S | D S | E S | S S
      ;
   
   
  
positive.code: argument
     : number
     | DOLLAR
     | REGISTER
     | name
     | string
     | ('(' expression ')')
  
negative.code:    | OFFSET expression
     ;
  
  register
     : REGISTER
     ;
  
  string
     : STRING
     ;
  