[texhax] the role of the tilde "~" in the set definition

[P. R. Stanley]

Hi folks
Here's another one taken from the Spivey text on the z (pronounced 
zed) language:
	\[ \{~p: PERSON | age(p) \geq 16~\} \]
	A simple set definition. Any idea what the "~" is supposed to do?
	
many thanks
Paul