Expose the child injector to the parent inject for a binding.

def expose(self, binder, interface, annotation=None):
        """Expose the child injector to the parent inject for a binding."""
        private_module = self
        class Provider(object):
            def get(self):
                return private_module.private_injector.get_instance(
                        interface, annotation)

        self.original_binder.bind(interface, annotated_with=annotation,
                                  to_provider=Provider)