How could we evaluate this indefinite double integral in the variables $x$ and $y$? $$\iint \frac{-e^x\sin{y}+e^y\sin{x}}{\phantom{-}e^x\cos{y}+e^y\cos{x}}dxdy$$
The argument of the double integral must be the derivative of a $y=f(x)$ function; being this quite strange, I think that in the double integration the $y$ could simplify.
For what a double integral is, see here.
In other words, what are the solutions $f$ to the following PDE?
$$f_{xy}=\frac{-e^x\sin{y}+e^y\sin{x}}{\phantom{-}e^x\cos{y}+e^y\cos{x}}$$