Elements of the theory of functions of a complex variable, with especial reference to the methods of Riemann. Authorized translation from the 4th German ... by George Egbert Fisher and Isaac J. Schwatt

The function ERROR is a standard function used to signal an error, which I'll discuss in greater detail in Chapter 19. Using those two functions, you can define a basic withdraw method that looks like this: (defmethod withdraw ((account bank-account) amount) (when (< (balance account) amount) (error "Account overdrawn.")) (decf (balance account) amount)) As this code suggests, the form of DEFMETHOD is even more like that of DEFUN than DEFGENERIC's is. The only difference is that the required parameters can be specialized by replacing the parameter name with a two-element list. The first element is the name of the parameter, and the second element is the specializer, either the name of a class or an EQL specializer, the form of which I'll discuss in a moment. The parameter name can be anything—it doesn't have to match the name used in the generic function, though it often will. This method will apply whenever the first argument to withdraw is an instance of bank-account. The second parameter, amount, is implicitly specialized on T, and since all objects are instances of T, it doesn't affect the applicability of the method ...»