anchor.diff:      : COMMENT
       ;
        
  +STRINGLITERAL
  +    : '"' STRING
  +    ;
  +
   STRING
       : [a-zA-Z] [a-zA-Z0-9_]*
       ;
  
positive.code:     : STRING
      ;
  
  stringliteral
      : '"' STRING
      ;
  
  value
      : stringliteral
      | expression
      | deref 
      ;
  
negative.code:     | label
      | setxy
      | make
      | procedureInvocation
      | ife
      | stop
  