anchor.diff:  
   [source,ANTLR4]
   ----
  -bitwise_and: expression '^' expression;
  +bitwise_xor: expression '^' expression;
   ----
   
   *Promotion*
  
positive.code:     implicit cast `def` to `int 7`;
      bitwise or `int 7` and `int 8` -> `int 15`;
      implicit cast `int 15` to `def` -> `def`;
      store `def` to `y`
negative.code: 
  [source,ANTLR4]
  ----
  bitwise_or: expression '|' expression;
  ----
  
  *Promotion*
  