DOC spelling
RESTRUCT restructure the layout to improve readability
RESTRUCT better layout to improve readability
BUGFIX fix the generator for prop_mod_inv

It could very easily get stuck when testing more than 100 times. For 100 times
it was less prone to fail, but maybe 1 out of 10 runs in travis failed due to
this problem.

The new procedure to generate coprimes generates first two non-overlapping lists
of prime factors, so we know for sure gcd is 1. This has the advantage that we
will be testing big numbers as well, with the previous generator it was very
unlikely that we could find a pair of big coprimes.
FUNCT add sel_lists:reduce/2
RESTRUCT use sel_lists:reduce
TEST tests for sel_lists:reduce/2