15.1.6 Generic Function Invocation Protocol

compute-applicable-methods-using-classes is not implemented.

compute-discriminating-function is implemented and returns the discriminator but:

Moreover add-method does not call compute-discriminating-function because this would be inefficient when doing multiple calls to add-method . Instead, compute-discriminating-function is called when the generic function is called.

LispWorks User Guide - 11 Mar 2008