Slant tiles over ...
And here is the definition of gradient.
Gradient Da[xixj] of ... is monomial xixj.
In particular, tangent bundle TB over B is identified with the cartesian product of ...
Shown on the left is the fiber over a flat tile. And all of these slant tiles are projected on the same flat tile. Each of the slant tiles assumes one of the three gradients shown on the right. For example, the gradient of a[x1x2] is x1x2 and so on.