Robyn Eckhardt writes: Back in February, with the help of a Thai friend, we spent several days nosing around Chiang Mai in an attempt to sniff out a definitive khao soi history. I won't claim that we succeeded, but we did turn up a few interesting tidbits.It's complicated. Much too complicated for 800 words. So for now, here's our khao soi primer, published in today's edition of Wall Street Journal Asia's 'Weekend Asia'. Next, the back story.