code: private function clearSummary($flags)
      {
          $return = null;
          $clearErrors = $flags & Debug::CLEAR_SUMMARY_ERRORS;
          if ($flags & Debug::CLEAR_SUMMARY) {
              $return = 'summary ('.($clearErrors ? 'incl errors' : 'sans errors').')';
              $curPriority = \end($this->data['groupPriorityStack']);  // false if empty
              foreach (\array_keys($this->data['logSummary']) as $priority) {
                  $entriesKeep = array();
                  if ($priority === $curPriority) {
                      $curDepth = 0;
                      foreach ($this->data['groupStacks'][$priority] as $group) {
                          $curDepth += (int) $group['collect'];
                      }
                      $entriesKeep = $this->debug->internal->getCurrentGroups(
                          $this->data['logSummary'][$priority],
                          $curDepth
                      );
                  } else {
                      $this->data['groupStacks'][$priority] = array();
                  }
                  $this->clearLogHelper($this->data['logSummary'][$priority], $clearErrors, $entriesKeep);
              }
          } elseif ($clearErrors) {
              $return = 'summary errors';
          }
          return $return;
      }
docs: Clear summary entries
  
  @param integer $flags flags passed to clear()
  
  @return string|null