Common Lisp the Language, 2nd Edition

28.1.6.4. Congruent Lambda-Lists for All Methods of a Generic Function

These rules define the congruence of a set of lambda-lists, including the lambda-list of each method for a given generic function and the lambda-list specified for the generic function itself, if given.

• Each lambda-list must have the same number of required parameters.

• Each lambda-list must have the same number of optional parameters. Each method can supply its own default for an optional parameter.

• If any lambda-list mentions &rest or &key, each lambda-list must mention one or both of them.

• If the generic function lambda-list mentions &key, each method must accept all of the keyword names mentioned after &key, either by accepting them explicitly, by specifying &allow-other-keys, or by specifying &rest but not &key. Each method can accept additional keyword arguments of its own. The checking of the validity of keyword names is done in the generic function, not in each method. A method is invoked as if the keyword argument pair whose keyword is :allow-other-keys and whose value is t were supplied, though no such argument pair will be passed.

• The use of &allow-other-keys need not be consistent across lambda-lists. If &allow-other-keys is mentioned in the lambda-list of any applicable method or of the generic function, any keyword arguments may be mentioned in the call to the generic function.

• The use of &aux need not be consistent across methods.

If a method-defining form that cannot specify generic function options creates a generic function, and if the lambda-list for the method mentions keyword arguments, the lambda-list of the generic function will mention &key (but no keyword arguments).