I don't know how to begin with this problem:

$\displaystyle

\int {{e^{ - x}}\tan {e^{ - x}}} dx

$

I've tried u substitution using $\displaystyle {u = {e^{ - x}}}

$ as well as integrations by parts using $\displaystyle u = \tan ({e^{ - x}})

$ and $\displaystyle dv = {e^{ - x}}

$, but those only seem to complicate the problem.

Any pointers?