1 files changed, 12 insertions, 4 deletions

diff --git a/spec/src/Thermoprint/Printout.hs b/spec/src/Thermoprint/Printout.hs
index 23e950a..c96573e 100644
--- a/spec/src/Thermoprint/Printout.hs
+++ b/spec/src/Thermoprint/Printout.hs
@@ -83,8 +83,12 @@ instance Monoid Block where
     | Seq.null xs = y
     | Seq.null ys = x
     | otherwise = NewlSep (xs <> ys)
-  (NewlSep xs) `mappend` y = NewlSep (xs |> y)
-  x `mappend` (NewlSep ys) = NewlSep (x <| ys)
+  (NewlSep xs) `mappend` y
+    | Seq.null xs = y
+    | otherwise = NewlSep (xs |> y)
+  x `mappend` (NewlSep ys)
+    | Seq.null ys = x
+    | otherwise = NewlSep (x <| ys)
   x `mappend` y = NewlSep $ Seq.fromList [x, y]
 
 instance Monoid Line where
@@ -93,8 +97,12 @@ instance Monoid Line where
     | Seq.null xs = y
     | Seq.null ys = x
     | otherwise = SpaceSep (xs <> ys)
-  (SpaceSep xs) `mappend` y = SpaceSep (xs |> y)
-  x `mappend` (SpaceSep ys) = SpaceSep (x <| ys)
+  (SpaceSep xs) `mappend` y
+    | Seq.null xs = y
+    | otherwise = SpaceSep (xs |> y)
+  x `mappend` (SpaceSep ys)
+    | Seq.null ys = x
+    | otherwise = SpaceSep (x <| ys)
   x `mappend` y = SpaceSep $ Seq.fromList [x, y]