add extend userlib func

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

snippets/userlib-tests.f

@@ -120,6 +120,13 @@
           (eq? (join l ".")
                "1.2.(3)"))))
 
+    ('extend extends sets'
+      (quote
+        (let ((s1 (1 2 3))
+              (s2 (4 5 6)))
+          (eq? (extend s1 s2)
+               (1 2 3 4 5 6)))))
+
      ;; add more test cases here
    ))
 

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)))