typo
Convert tikz diagrams to xymatrix (both versions visible), reformat ACs

This version contains both, the TiKZ and the xypic versions of diagrams,
for easy comparison, see issue #114.

Also, I reformatted the statements of the axiom of choice so that they
use up less space. They were breaking the page formatting.
Convert TiKZ diagrams to xypic, closing issue #114