Add special case in juxtaposeDefault for empty envelope (see #37)

In particular, juxtaposeDefault is now the identity on the second
object if either one has an empty envelope.  In particular this means
that mempty is now an identity element for beside and friends.

src/Graphics/Rendering/Diagrams/Juxtapose.hs

@@ -18,6 +18,7 @@ module Graphics.Rendering.Diagrams.Juxtapose
        ( Juxtaposable(..), juxtaposeDefault
        ) where
 
+import           Data.Functor ((<$>))
 import qualified Data.Map as M
 import qualified Data.Set as S
 
@@ -40,11 +41,15 @@ class Juxtaposable a where
   juxtapose :: V a -> a -> a -> a
 
 -- | Default implementation of 'juxtapose' for things which are
---   instances of 'Enveloped' and 'HasOrigin'.
+--   instances of 'Enveloped' and 'HasOrigin'.  If either envelope is
+--   empty, the second object is returned unchanged.
 juxtaposeDefault :: (Enveloped a, HasOrigin a) => V a -> a -> a -> a
-juxtaposeDefault v a1 a2 = moveOriginBy (v1 ^+^ v2) a2
-  where v1 = negateV (envelopeV v a1)
-        v2 = envelopeV (negateV v) a2
+juxtaposeDefault v a1 a2 =
+  case (mv1, mv2) of
+    (Just v1, Just v2) -> moveOriginBy (v1 ^+^ v2) a2
+    _                  -> a2
+  where mv1 = negateV <$> envelopeVMay v a1
+        mv2 = envelopeVMay (negateV v) a2
 
 instance (InnerSpace v, OrderedField (Scalar v)) => Juxtaposable (Envelope v) where
   juxtapose = juxtaposeDefault