code: public function hasExtension($class)
      {
          $class = ltrim($class, '\\');
          if (!isset($this->extensionsByClass[$class]) && class_exists($class, false)) {
              // For BC/FC with namespaced aliases
              $class = new ReflectionClass($class);
              $class = $class->name;
          }
  
          if (isset($this->extensions[$class])) {
              if ($class !== get_class($this->extensions[$class])) {
                  @trigger_error(sprintf('Referencing the "%s" extension by its name (defined by getName()) is deprecated since 1.26 and will be removed in Twig 2.0. Use the Fully Qualified Extension Class Name instead.', $class), E_USER_DEPRECATED);
              }
  
              return true;
          }
  
          return isset($this->extensionsByClass[$class]);
      }
docs: Returns true if the given extension is registered.
  
  @param string $class The extension class name
  
  @return bool Whether the extension is registered or not