Add a few tests for sets which reflect new changes.

tests/snippets/test_sets.py

@@ -0,0 +1,16 @@
+
+empty_set = set()
+non_empty_set = set([1,2,3])
+
+assert 1 in non_empty_set
+assert 4 not in non_empty_set
+
+# TODO: Assert that empty aruguments raises exception.
+non_empty_set.add('a')
+assert 'a' in non_empty_set
+
+# TODO: Assert that empty arguments, or item not in set raises exception.
+non_empty_set.remove(1)
+assert 1 not in non_empty_set
+
+# TODO: Assert that adding the same thing to a set once it's already there doesn't do anything.