mmult :: forall m n k a. (CRing a, KnownNat m, KnownNat n, KnownNat k) =>
    M m k a -> M k n a -> M m n a
mmult x y = tabulate (\(i,j) -> row i x <.> col j y)
mmult :: forall m n k a. (CRing a, KnownNat m, KnownNat n, KnownNat k) =>
    M m k a -> M k n a -> M m n a
mmult x y = tabulate (\(i,j) -> row i x <.> col j y)
mmult x y = tabulate (\(i,j) -> row i x <.> col j y)