anchor.diff:  
   OF
       :   'of'
  +    ;
  +
  +OUT
  +    :   'out'
       ;
   
   SUBTYPE
  
positive.code:     ;
  
  variance
      : 'in' | 'out'
      ;
      
  dimensionalTypeParameter
  
negative.code:     SEQ_ARG;
      NIL;
      RET_STMT;
      BLOCK;
      THROW_STMT;
      RETRY_STMT;
      TRY_BLOCK;
  