code: def register_instance(self, class_type, instance, register_as=None):
          
          registration = _InstanceRegistration(instance)
          self._register(class_type, registration, register_as)
docs: Registers the given instance (already created).
          :param class_type: The class type.
          :param instance: The instance to register.
          :param register_as: The types to register the class as, defaults to the given class_type.