Pure properties

[address@hidden]

Hey all,

I think I've asked this question before, but I can't find the response - sorry 
if I'm repeating myself!

Could someone please send me a paragraph-long response about what pure 
properties are and how/when/why they're used?  The more sequential, the better 
(i.e. pure properties are defined in X for the purpose of doing Y when lilypond 
hits function Z).  git grep shows the word `pure' all over the place, and I 
feel kinda stupid not knowing what pure means in the LilyPond lingo. I'm 
writing a patch right now that allows LilyPond to do multiple passes through a 
score to fix several spacing issues I've run into, and before taking it on 
seriously I'd like to fill this gap in my knowledge of the program.

Cheers,
Mike