add extend userlib func

Signed-off-by: Ava Affine <ava@sunnypup.io>

snippets/userlib.f

@@ -156,3 +156,9 @@ interspersing a provided delimiter in between elements'
               (string elem) 
               (concat res delim elem)))
     list))
+
+(def extend
+'adds all elements in set2 to set1'
+  (set1 set2)
+  (reduce (lambda (res elem) (cons res elem))
+          (prepend set1 set2)))