implement add operation

src/Language/Lambda/Parser.hs

@@ -53,6 +53,7 @@ keyword :: String -> Parser ()
 keyword = void . lexeme . string
 
 doOperation "" = ""
+doOperation (x:'+':y:xs) = ubahKeChurch x ++ ubahKeChurch '+' ++ ubahKeChurch y ++ doOperation xs
 doOperation (x:xs) = ubahKeChurch x ++ doOperation xs
 
 ubahKeChurch :: Char -> [Char]