code: public function registerFreeProcess(ProcessDetails $processDetails) {
  		$pid = $processDetails->getPid();
  		if ($this->hasProcess($pid) === FALSE) {
  			throw new \InvalidArgumentException(sprintf('Could not register free process. Process (%d) not in list.', $processDetails->getPid()), 1400761296);
  		}
  		$this->freeProcessIds[$pid] = $pid;
  
  		return $this;
  	}
docs: Register a ProcessDetails as free
  
  @param ProcessDetails $processDetails
  @throws \InvalidArgumentException
  @return ProcessDetailsCollection