matrix mult
write:
 $z_k <-B- y_j <-A- x_i$
  $c_{ki} = \sum b_{kj} * a_{ji}$
$c_{ki}$ is "how $z_k$ changes given $x_i$"
...and the answer is:
a given $x_i$ affects all the $y_j$ (via $a_{ji}$),
which combine (via $b_{kj}$) to affect $z_k$.

It's really just "gather like terms"
  (equiv, same picture as multi-variable chain rule)
