defgeneric

(defgeneric name (param...) option...)

Defines a generic function dispatching on its first argument and returns the name symbol. Methods are added with defmethod; calling the generic runs the most specific method matching the first argument, and calling it with no matching method signals an error. A defgeneric is optional — the first defmethod implicitly creates the generic — but declares the lambda list every method must match. The generic function is an ordinary function, so #'name and funcall work.

Lite subset: the lambda list takes required parameters only (&optional/&rest/&key are errors), and the only supported option is (:documentation "...") (recorded and ignored) — (:method ...), :method-combination and the rest are errors.

Calling a generic that has no applicable method signals an error (No applicable method: g), so it is shown here statically rather than as a runnable example:

(defgeneric g (x))
(g 1)