diff --git a/2021/day20.hs b/2021/day20.hs
index ccf8cf7..ce849dd 100644
--- a/2021/day20.hs
+++ b/2021/day20.hs
@@ -30,7 +30,8 @@ next :: Algorithm -> Image -> Image
 next algo (Img def diff) = Img (newDefault algo def) newDiff where
     nonDefault = foldr (\(x,y) -> insertManyWith (++) (neighs x y)) M.empty diff
     neighs x y = map (\i -> ((x-1 + i `mod` 3, y-1 + i `div` 3), [i])) [0..8]
-    calcNonDef = M.filter (/= newDefault algo def) $ M.map (\ns -> algo A.! binToDec (nonDefToNum ns)) nonDefault
+    known = M.map (\ns -> algo A.! binToDec (nonDefToNum ns)) nonDefault
+    calcNonDef = M.filter (/= newDefault algo def) known
     nonDefToNum ns = map (\i -> if i `elem` ns then not def else def) [0..8]
     newDiff = M.keys calcNonDef