I have been unable to find a nice article on the essence of callback handlers. So, for instance, it is introduced on p 378 of Kochan 2.0. Eg -(BOOL) removeFileAtPath:handler: I would just like to understand why passing "nil" as an argument returns "NO" if the return type is BOOL, and what else one could do, if "nil" is not passed. Not in detail, but just in very general principles. Thank you in advance.