Comment #16 on issue 1654 by asmeurer: subs() doesn't substitute lower
derivatives into higher order ones
http://code.google.com/p/sympy/issues/detail?id=1654
I think we should do one of the following:
1. Keep track of if the Derivative was instantiated with diff or Derivative,
2. Use a different method to "differentiate" between when it should sort
args or not
3. Just assume that a Derivative object was instantiated in unevaluated
form.
Of these, I think 1. is the best. I don't like the idea of guessing based
on if the args are sorted or not.
Also consider in your tests the cases diff.subs(Derivative) and
Derivative.subs(diff).
By the way, your bug reminds me of issue 1888 and issue 1604. In those
issues, integrate() was giving strange answers for Derivative objects, and
it was because Derivative.subs() was evaluating the derivative, removing
the information that it depended on some variables after some dummy
variable substitutions in the integration algorithm. This was fixed by
issue 2049, which made Derivative.subs always return an unevaluated
Derivative.
So I would check that you aren't evaluating unevaluated Derivative objects
with your subs routine, even in (actually especially in) trivial cases.
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