integral transform

[Chris Smith]

There is a question here about integrating `cos(x)**2` wrt `sin(x)` -- which is something that is no longer permitted in current versions of SymPy and this is not a permitted transform since solution of `u - sin(x)` does not yield a unique solution. Is this something that SymPy should be handling?

/c

[Chris Smith]

As indicated [here](https://www.physicsforums.com/threads/integrating-wrt-a-function.384189/), in such cases one may compute `integrate(f(x), g(x))` as `integrate(f(x)*g(x).diff(x), x)`