In a space with a given metric $g_{a b}$, a covariant derivative is defined by $T^a{ }_{\| n}=T^a{ }_{, n}+D_{n m}^a T^m$. Calculate $f_{, n m}-f_{, m \| n}$ and show that $S_{n m}^a=D_{n m}^a-D_{m n}^a$ is a tensor! Can $S_{n m}^a$ be determined by demanding $g_{a b \| n}=0$ ?