The problem is that Subs has wrong _hashable_content()
I will send a pull request with a fix.
However I am still interested to know how this was never before
encountered. What was so special about this expression that it raised
an error?

On 22 July 2012 01:02, [email protected]
<[email protected]> wrote:
>>>>  (g(f(x))*Subs(Derivative(1, x), (x,), (1,))).diff(x)
> TypeError: object of type 'Derivative' has no len()
>
> Simplifying the expression any further makes the error go away (for
> instance removing the call to g).
>
> What perplexes me is how unremarkable the expression is. What can
> cause the error? Any idea?

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