gistfile1.lisp

 (defun example (&rest maps)
      (format nil "~:[<none>~;~:*~{~A~^, ~}~]"
              (sort (remove-duplicates
                     (loop for map in maps nconc
                           (loop for key being the hash-key of map collect key))
                     :test 'equal)
                    'string<)))