@@ -323,6 +323,20 @@

                           it

                           (alexandria:iota it-length))))))))

+(defun-ct group-by (fn &key (test 'equal))

+  (lambda (seq)

+    (let ((groups (make-hash-table :test test)))

+      (map nil

+           (lambda (it)

+             (push it

+                   (gethash (funcall fn it)

+                            groups)))

+           seq)

+      (mapcar (lambda (it)

+                (cons (car it)

+                      (reverse (cdr it))))

+              (alexandria:hash-table-alist groups)))))

+

 #+nil

 (defmacro <> (arity &rest funs)

   (let ((arg-syms (loop repeat arity collect (gensym))))