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[...] 2. A way to define a function signature type. For instance, a type that represents the class of all functions that take two number arguments. [...] (deftype functions-that-take-two-number-arguments () (quote (function (number number) t))) examples: CL-USER> (deftype functions-that-take-two-ar...

afair, there was discussions on related matter at cll:
http://groups.google.com/groups?as_q=ma ... .lang.lisp
http://groups.google.com/groups?as_q=ma ... .lang.lisp