code: public function has($id, $checkInstance = false)
      {
          return $checkInstance ? isset($this->_components[$id]) : isset($this->_definitions[$id]);
      }
docs: Returns a value indicating whether the locator has the specified component definition or has instantiated the component.
  This method may return different results depending on the value of `$checkInstance`.
  
  - If `$checkInstance` is false (default), the method will return a value indicating whether the locator has the specified
  component definition.
  - If `$checkInstance` is true, the method will return a value indicating whether the locator has
  instantiated the specified component.
  
  @param string $id component ID (e.g. `db`).
  @param bool $checkInstance whether the method should check if the component is shared and instantiated.
  @return bool whether the locator has the specified component definition or has instantiated the component.
  @see set()