On page 287, A. van den Essen says:
Furthermore one can show that it suffices to prove JC for all $n \geq 2$ and for all $F$'s of the form: $F=(l_1,\ldots,l_r,x_{r+1}+M_{r+1},\ldots,x_n+M_n)$ where each $l_i$ is linear and each $M_j$ is a monomial.
Please, where a proof of this result can be found? (I remember I have once found an article with this result, but not I am not able to find it again).
Thank you!