If $T : A\to X$ we say that a square $\vf_2 : US\To VR$ is $T$-exact if the grid is exact at $X$, and that $T$ is an opaque functor if the square is $T$-exact.
If $T : A\to X$ we say that a square $\vf_2 : US\To VR$ is $T$-exact if the grid is exact at $X$, and that $T$ is an opaque functor if the square is $T$-exact.