def redefine(self, type, fn):
return self.redefine_many({type: fn})<EOL>
Produce a new checker with the given type redefined.

Arguments:

    type (str):

        The name of the type to check.

    fn (callable):

        A function 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.

Returns:

    A new `TypeChecker` instance.