Comment #31 on issue 1816 by Vinzent.Steinberg: Adding partial derivatives and taking derivatives with respect to functions
http://code.google.com/p/sympy/issues/detail?id=1816
Sorry, I don't get your point. Why can't we consider diff(x'(t), x(t)) as diff(x', x)? Let's say they are just partial derivatives. In your example, you differentiate wrt x' (which you call r), but suddenly you have x'(t) and take the derivative of it. This is a contradiction to x' != x'(t), isn't it?
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