Add more tests.

tests.el

@@ -1,6 +1,27 @@
 (require 'ert)
 (require 'bang)
 
+(ert-deftest difference ()
+  "`!difference' returns a new list of only elements in list1 that are not in list2."
+  (should (equal (!difference '() '()) '()))
+  (should (equal (!difference '(1 2 3) '(4 5 6)) '(1 2 3)))
+  (should (equal (!difference '(1 2 3 4) '(3 4 5 6)) '(1 2))))
+
 (ert-deftest intersection ()
   "`!intersection' returns a new list of only elements that are in both given lists."
+  (should (equal (!intersection '() '()) '()))
+  (should (equal (!intersection '(1 2 3) '(4 5 6)) '()))
   (should (equal (!intersection '(1 2 3 4) '(3 4 5 6)) '(3 4))))
+
+(ert-deftest uniq ()
+  "`!uniq' returns a new list of only unique elements."
+  (should (equal (!uniq '()) '()))
+  (should (equal (!uniq '(1 2 2 4)) '(1 2 4))))
+
+(ert-deftest contains-p ()
+  "`!contains-p' returns t if the list contains the element."
+  (should (!contains-p '(1 2 3) 1))
+  (should (!contains-p '(1 2 3) 2))
+  (should (not (!contains-p '() '())))
+  (should (not (!contains-p '() 1)))
+  (should (not (!contains-p '(1 2 4) 3))))