anchor.diff:     | expression
      ;
   
  -real_literal
  -   : DIGIT+ '.' DIGIT+
  -   ;
  -
  -char_literal
  -   : 'a'
  -   ;
  -
  -string_literal
  -   : STRING
  -   ;
  -
  -STRING
  -   : '"' .*? '"'
  -   ;
  -
   name
      : IDENTIFIER
      ;
  
positive.code:    ;
  
  equates
     : idn '=' 'equates' (parms where?)? equate equate* 'end' idn
     ;
  
  guardian
  
negative.code:    ;
  
  parms
     : (parm (',' parm)*)
     ;
  
  parm
  