custom-set: Corrected Function Names to match Test (exercism#1590)

vigneshhari · cmccandless · commit e6ed992777e4 · 2018-11-13T10:39:36.000-07:00
Fixes exercism#1589
diff --git a/exercises/custom-set/custom_set.py b/exercises/custom-set/custom_set.py
@@ -23,8 +23,8 @@ def add(self, element):
     def intersection(self, other):
         pass
 
-    def difference(self, other):
+    def __sub__(self, other):
         pass
 
-    def union(self, other):
+    def __add__(self, other):
         pass
