Question: Total differentials

I fail to understand from the manual how to do total differentiation.

e.g let's say I have a trivial transform defined as

u=t(x,y)-s(x,y);

then I should get

du=diff(t,x)dx+diff(t,y)dy ...etc

Apparently the operator D_Dx should be able to perform the total derivative, but somehow I cant see this is possible.

What do I miss ?

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