anchor.diff:  
     visitGetMutableVariable(cps_ir.GetMutableVariable node) {
       String variable = names.name(node.variable.definition);
  -    return '${node.runtimeType} $variable';
  +    return 'GetMutableVariable $variable';
     }
   }
   
  
positive.code:     String dummy = names.name(node);
      String variable = names.name(node.variable.definition);
      String value = formatReference(node.value);
      printStmt(dummy, '${node.runtimeType} $variable := $value');
      visit(node.body);
    }
  
  
negative.code:   }
  
    visitMutableVariable(cps_ir.MutableVariable node) {
      return "MutableVariable ${names.name(node)}";
    }
  
    visitContinuation(cps_ir.Continuation node) {
  