Buttercup knows whether or not Westley is lying. She promises that if Westley is lying, she will give you a cookie. Buttercup always keeps her promises.

Suppose she does not give you a cookie; what can you conclude?
? Westley is lying. Westley is not lying. Not enough information to determine.

Suppose she gives you a cookie; what can you conclude?
? Westley is lying. Westley is not lying. Not enough information to determine.