code: public function setAssignee(?string $assignee)
      {
          if (null !== $assignee) {
              if (!preg_match('/^[a-z]+$/i', $assignee)) {
                  throw new DomainException('Assignee is not valid.');
              }
          }
  
          $this->assignee = $assignee;
  
          return $this;
      }
docs: Set assignee
  
  @param string $assignee
  @return object
  @throws \DomainException