ANSI Common Lisp   3 Evaluation and Compilation   3.4 Lambda Lists

3.4.8 Deftype Lambda Lists

A deftype lambda list has the same syntax as a macro lambda list, and can therefore contain the lambda list keywords as a macro lambda list.

A deftype lambda list differs from a macro lambda list only in that if no init-form is supplied for an optional parameter or keyword parameter in the lambda-list, the default value for that parameter is the symbol * (rather than nil).