code: public function compute($tags, $target)
      {
          // cannot compute a duration across different targets
          if (!$this->onlyOneFlag($target)) {
              throw new InconsistentCriteriaException('Cannot compute a duration across different targets!');    
          }
          
          return $this->timeLine->compute($tags, $target);
      }
docs: Computes the total duration represented by the filtered TimePoints
  @param string|array $tags A tag or a list of tags to filter
  @param int $target The type of target TimePoint to filter
  @return float Returns the total computed duration
  @throws TimeException