Support derivate like d(f(x))/d(2*x) (Chain Rule)

[latot]

Hi all, well i think can be good idea implement this, basically is this:

>>> b=sympy.S("x")
>>> b.diff(2*b)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib64/python2.7/site-packages/sympy/core/expr.py", line 2881, in diff
    return Derivative(self, *new_symbols, **assumptions)
  File "/usr/lib64/python2.7/site-packages/sympy/core/function.py", line 1112, in __new__
    Can\'t calculate %s%s derivative wrt %s.''' % (count, ordinal, v)))
ValueError: 
Can't calculate 1st derivative wrt 2*x.

i this case should be:

d(b)/d(2*x)=(d(b)/d(x))/(d(2*x)/d(x))

Thx. Cya.

[smichr]

The reasons for not supporting differentiation wrt expressions is given in the docstring of Derivative.