anchor.diff:  ;
   
   rule_
  -    : lhs '::=' rhs
  +    : lhs ASSIGN rhs
       ;
   
   lhs
  
positive.code:     ;
  
  alternative
      : term ( '|' term)*
      ;
  
  term
  
negative.code:     ;
  
  optional
      : REND alternative+ LEND
      ;
  
  zeroormore
      : RBRACE alternative+ LBRACE
      ;
  
  oneormore
      : RPAREN alternative+ LPAREN
      ;
  
  text
  