Jacket else

I added the discussed function `jacket-else()` to the code base and added the documentation.
Going through the code I found some places where I thought I should change something else.
The most unsure change was done in my first commit where I removed unnecessary code. It was not documented and seamed to me as a leftover from a former concept.
I hope those changes do not interfere with your ideas for your project and would be glad to see them accepted.
If you need some Changes that I Should still do, please let me know.

Regards, Hannes