>
outdegree (buildG (0,-1) []) == array (0,-1) []
outdegree (buildG (0,2) [(0,1), (1,2)]) == array (0,2) [(0,1),(1,1),(2,0)]
outside :: Prism s t a b -> Lens (t -> r) (s -> r) (b -> r) (a -> r)