diff -up sized-types-0.3.4.0/Data/Sized/Sparse/Matrix.hs.fix sized-types-0.3.4.0/Data/Sized/Sparse/Matrix.hs
--- sized-types-0.3.4.0/Data/Sized/Sparse/Matrix.hs.fix	2012-06-02 15:00:30.688853704 +0530
+++ sized-types-0.3.4.0/Data/Sized/Sparse/Matrix.hs	2012-06-02 15:03:05.782857244 +0530
@@ -47,7 +47,7 @@ prune d sm@(Matrix d' m) | d == d'   = M
 sparse :: (Size ix, Eq a) => a -> M.Matrix ix a -> Matrix ix a
 sparse d other = Matrix d (Map.fromList [ (i,v) | (i,v) <- M.assocs other, v /= d ])
 
-mm :: (Size m, Size n, Size m', Size n', n ~ m', Num a) => Matrix (m,n) a -> Matrix (m',n') a -> Matrix (m,n') a
+mm :: (Size m, Size n, Size m', Size n', n ~ m', Num a, Eq a) => Matrix (m,n) a -> Matrix (m',n') a -> Matrix (m,n') a
 mm s1 s2 = Matrix 0 mp
   where
 	mp = Map.fromList [ ((x,y),v)
@@ -91,5 +91,4 @@ transpose (Matrix d m) = Matrix d (Map.f
 
 zipWith :: (Size x) => (a -> b -> c) -> Matrix x a -> Matrix x b -> Matrix x c
 zipWith f m1 m2 = pure f <*> m1 <*> m2 
-	
-	
\ No newline at end of file
+