Note, I haven't used C in 15 years, have never looked at memcached code and in fact the last time I used memcached was before redis even existed.
It took me ~26 mins which included reading the article, googling the memcached repo, downloading all dependencies, compiling a first build, grepping for references to incr and adding mult with support for negative arguments, refactoring the guts of incr into two functions to support the changes, compiling, fixing a few compiler bugs then testing, given a few more minutes I'd probably be able to get it PR ready with a full test suite.
