code: public function getRuntime($class)
      {
          if (isset($this->runtimes[$class])) {
              return $this->runtimes[$class];
          }
  
          foreach ($this->runtimeLoaders as $loader) {
              if (null !== $runtime = $loader->load($class)) {
                  return $this->runtimes[$class] = $runtime;
              }
          }
  
          throw new Twig_Error_Runtime(sprintf('Unable to load the "%s" runtime.', $class));
      }
docs: Returns the runtime implementation of a Twig element (filter/function/test).
  
  @param string $class A runtime class name
  
  @return object The runtime implementation
  
  @throws Twig_Error_Runtime When the template cannot be found