code: public function sumByKey($key = false)
      {
          $collection = $this->getIterator();
          $sum = 0;
          foreach ($collection as $result) {
              if ($key && isset($result->$key)) {
                  $sum += $result->$key;
              }
              else if ($key && isset($result[$key])) {
                  $sum += $result[$key];
              }
              else {
                  $sum += $result;
              }
          }
          return $sum;
      }
docs: Returns the sum of the contents of the Container.
  If given a key, will sum that $key->value.
  Obviously it's important to have some consistency in the types of resources you
  are storing or else the results might not make much sense or could crash.
  
  @param key A key on which to sum the when looking at the contents of this Container.
  @return A number.