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

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.)

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!

The number of this PEP was intentionally chosen - as PEP index doesn't have to be continuous -leaving Tim with the freedom to choose whatever number he wanted.

Now the question comes to why the number 20 was chosen, if 19 aphorisms were written, why he didn't name his proposal as PEP19? - This is where Zen started to be involved.

In the Zen-fluenced Japanese sushi restaurants, customers have the option to choose Omakase which means "I'll leave it up to you", in return (not typical but sometimes) the sushi chef will ask the customers to choose the last piece of sushi - either exploring a new fish or aftertast-ing a previous one - conceptual-wise it correlates to what @Jeff Walden mentioned in his answer where people have the opportunity to provide their own addition to complete the set.

After all it's all pure speculation.