The Zen of Python distils the guiding principles for Python into 20 aphorisms but lists only 19. What's the twentieth?

I had the opportunity to ask Guido about this recently. According to him, this is "some bizarre Tim Peters in-joke". That, and/or (still according to him) it's an opportunity for people to provide their own addition (as largely is happening in the answers to this question :-) ).

It has to be SIGNIFICANT WHITESPACE, of course!

Rule number 20: there is no rule #20.