1000 answers and 20K points. Time to quit, again. (April 30th 2016)

One reason ... the people who think that this site's purpose is to build an archive of knowledge, and that giving the OP a useful answer is merely a "nice side-effect" of this process. I don't share that view.

Another reason ... the lecturings I have received about the nature of mathematics and society. And, worst of all, from graduate students, sometimes. I'm much too old to value the pontifications of folks with no real life experience outside of a classroom.

So, once again, I'm gone. 再见.