diff --git a/src/Hanafuda.hs b/src/Hanafuda.hs
index e1fe004..a19357e 100644
--- a/src/Hanafuda.hs
+++ b/src/Hanafuda.hs
@@ -13,6 +13,7 @@ module Hanafuda (
     , match
     , packOfCards
     , remove
+    , sameMonth
     , shuffle
     , size
   ) where
@@ -111,8 +112,8 @@ intersection = portBinary (.&.)
 difference :: Pack -> Pack -> Pack
 difference = portBinary (\a b -> a `xor` (a .&. b))
 
-sameMonth :: Card -> Pack
-sameMonth card = Pack $ 0xf `shift` (fromEnum card .&. 0xfc)
+sameMonth :: Card -> Pack -> Pack
+sameMonth card (Pack p) = Pack $ (0xf `shift` (fromEnum card .&. 0xfc)) .&. p
 
 cards :: [Card]
 cards = [Pine0 .. Phoenix]
@@ -130,7 +131,7 @@ shuffle l =
 
 match :: Card -> Pack -> Either String (Pack, [Card])
 match card pack =
-  let sameMonthCards = sameMonth card `intersection` pack in
+  let sameMonthCards = sameMonth card pack in
   case size sameMonthCards of
     0 -> Right (add pack card, [])
     1 -> Right (difference pack sameMonthCards, card : cardsOfPack sameMonthCards)