Comment #14 on issue 1620 by asmeurer: Allow derivatives of unknown functions evaluated at a point
http://code.google.com/p/sympy/issues/detail?id=1620
We can still make an operator, D, which acts like D[1, 2](f)(x, y), but also allow the syntax D(f, orders=(1, 2))(x, y), and it would satisfy the invariant.
It seems to me that having an operator, D, would allow more power. And if we're going to build that syntactic sugar anyway (I think we should), why not just make that the way that it is actually implemented.
By the way, in Maple, D is even more powerful than what I said above, because it allows symbolic entries on the variable (D[i, j](f)), and even symbolic orders of the derivative (D[$(1, n), $(2, m)](f)).
The Maple help page (see comment 7) also reminded me that this should work with compositions of functions, like D(f(g)).
I suppose that D(f)(x) would be the same as f(x).diff(x). Maple keeps the two separate and has a function to convert between the two.
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