code: public R having() {
      if (whereStack == null) {
        whereStack = new ArrayStack<>();
      }
      // effectively putting having expression list onto stack
      // such that expression now add to the having clause
      whereStack.push(query.having());
      return root;
    }
docs: Start adding expressions to the having clause when using @Aggregation properties.
  
  <pre>{@code
  
  new QMachineUse()
  // where ...
  .date.inRange(fromDate, toDate)
  
  .having()
  .sumHours.greaterThan(1)
  .findList()
  
  // The sumHours property uses @Aggregation
  // e.g. @Aggregation("sum(hours)")
  
  }</pre>