code: protected function resolve($id)
      {
          $entry = null;
  
          isset($this->instances[$id]) && $entry = $this->instances[$id];
  
          isset($entry) === false && $entry = $this->injector->make($id);
  
          return $entry;
      }
docs: Resolves the specified identifier to an instance.
  
  @param  string $id
  @return mixed