code: public function start() : Stopwatch
      {
          Preconditions::checkState(!$this->isRunning, 'This stopwatch is already running.');
          $this->isRunning = true;
          $this->startTick = $this->ticker->read();
          return $this;
      }
docs: Starts the stopwatch.
  
  @return $this
  @throws IllegalStateException if this Stopwatch is already running