code: public function has($key) : bool
      {
          try {
              $this->get($key);
              return true;
          } catch (NotFoundExceptionInterface $e) {
              return false;
          }
      }
docs: Checks if a dependency exists identified by $key. Note that this also
  resolves the dependency internally, so any dependencies on $key must also
  be resolvable for this to return true.
  
  Note `$key` is not type hinted as it would violate the
  `Psr\Container\ContainerInterface`.
  
  @param string $key The unique identifier for the dependency.
  @return bool True if $key identifies a known dependency, else false.