add Examples text to docstring and shorten line
make Abs args canonical wrt sign
add symbolic mod handling so Mod(4*integer, 4) == 0
modify hyperexpand mod1 function
add new Min-replacement pattern to compute_cdf

If you don't do this you might get alternate results

https://travis-ci.org/smichr/sympy/jobs/19850479 shows the failure in
a run where 3 of the 4 python versions passed but one failed.
rework assumptions watching for 0