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

Question

From PEP 20, The Zen of Python:

Long time Pythoneer Tim Peters succinctly channels the BDFL's guiding principles for Python's design into 20 aphorisms, only 19 of which have been written down.

What is this twentieth aphorism? Does it exist, or is the reference merely a rhetorical device to make the reader think?

(One potential answer that occurs to me is that "You aren't going to need it" is the remaining aphorism. If that were the case, it would both exist and act to make the reader think, and it would be characteristically playful, thus fitting the list all the better. But web searches suggest this to be an extreme programming mantra, not intrinsically Pythonic wisdom, so I'm stumped.)

Answer 1

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 :-) ).

Answer 2

It has to be SIGNIFICANT WHITESPACE, of course!