On Sun, Dec 15, 2013 at 11:35:43PM -0800, Aaron Meurer wrote:
>    [1]Here it is. Notice the workarounds that are needed because f() doesn't
>    work. It makes sense for Python functions, so it should make sense for
>    SymPy functions.

Python functions are not pure.  SymPy functions - usual mathematical
functions (no side-effects, etc).