lispdoc - results for union

(union list1 list2 &key key (test #'eql testp) (test-not nil notp))

Function: Return the union of LIST1 and LIST2.

Example:

(defun debug (&rest ids)
  "Start dbg output on the given ids."
  (setf *dbg-ids* (union ids *dbg-ids*)))

Mentioned in:
CLtL2 - 15.5. Using Lists as Sets
HyperSpec - Function UNION, NUNION
On Lisp - Recursion on Cdrs (Macros Returning Functions)
PCL - destructuring bind
PCL - sets
PCL - what frames do you actually need