-- | A prism which focuses on the left branch of an Either
_Left :: Prism (Either a x) (Either b x) a b
_Left = rmap (either (Left <$>) $ pure . Right) . left'
_Left :: Prism (Either a x) (Either b x) a b
_Left = rmap (either (Left <$>) $ pure . Right) . left'
_Left = rmap (either (Left <$>) $ pure . Right) . left'