ANSI Common Lisp 7 Objects 7.1 Object Creation and Initialization
7.1.2 Declaring the Validity of Initialization ArgumentsInitialization arguments are checked for validity in each of the four situations that use them. An initialization argument may be valid in one situation and not another. For example, the system-supplied primary method for make-instance defined for the class standard-class checks the validity of its initialization arguments and signals an error if an initialization argument is supplied that is not declared as valid in that situation.
There are two means for declaring initialization arguments valid.
The set of valid initialization arguments for a class is the set of valid initialization arguments that either fill slots or supply arguments to methods, along with the predefined initialization argument :allow-other-keys. The default value for :allow-other-keys is nil. Validity checking of initialization arguments is disabled if the value of the initialization argument :allow-other-keys is true.