It seems he recently made an exception to the rule by collaborating with Stephan Pastis, author of the ‘Pearls Before Swine‘ strip. Mr. Pastis provides details of the collaboration on his blog. Here’s a sample of the joint effort, reduced in size to fit this blog:
It’s an interesting and amusing story. Now to see if Mr. Pastis follows through on his promise to set his hair on fire . . .