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