def METHOD_NAME(self, type: str, fn) -> TypeChecker:
    """
    Produce a new checker with the given type redefined.
    Arguments:
        type:
            The name of the type to check.
        fn (collections.abc.Callable):
            A callable taking exactly two parameters - the type
            checker calling the function and the instance to check.
            The function should return true if instance is of this
            type and false otherwise.
    """
    return self.redefine_many({type: fn})