PI is a variable.

http://answers.yahoo.com/question/index?qid=20080708162347AAh4RJ4
A variable is an unknown

but no one ever calculated or seen value of Pi, so Pi must be unknown. Besides, even on different SBCL implementations Pi has different values.

Anyone who seriously cares about using τ instead of π is a fucking retard with too much time on their hands. It's a pointless idea, even more pointless than metric time or gradians. Remove yourself from the company of thinking people, faggots, because you are only wasting their time.