Comment #8 on issue 1435 by renato.c...@gmail.com: f(x, y).diff(x, y) - f(x, y).diff(y, x) should simplify to zero
http://code.google.com/p/sympy/issues/detail?id=1435
Although the fix above is satisfactory for me, I thought that sorting args could confuse people. In master `f(x, y).diff(y, x)` gives `D(f(x, y), y, x)`, but with the patch, it gives `D(f(x, y), x, y)`.
It's possible to define Derivative.__eq__() so that `D(f(x, y), y, x) == D(f(x, y), x, y)` without sorting the args. But then we would have printing issues due to cache: if you print the expression in a certain order, than later prints the same derivative in a different order, the printed result will be the first, not your last input.
That's all to say: there's an alternative fix that rids us from a possible nuisance and yields another, maybe worse.
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