How do I define `org-agenda-cmp-user-defined'?

[Alan Schmitt <alan.schmitt@polytechnique.org>]

Hello,

I'm working on a way to schedule reviews of projects, and I would like
to have them ordered by age in an agenda view. It seems that I want to
write my own `org-agenda-cmp-user-defined' function to do so.
Unfortunately I cannot find an example of such a function.

More precisely, given this:

,-----------------------------------------------------------------
| (defun org-review-schedule-compare (a b)                        
|   "Compares the date of scheduled review for the two agenda     
|   entries. Returns +1 if A has been scheduled for longer, nil if
|   they have been scheduled for the same length of time, and -1  
|   otherwise."                                                   
|   ())                                                           
`-----------------------------------------------------------------

how can I access a property of `a' and `b'? (The documentation tells me
they are "agenda entries", but it's not clear how I can access their
information.)

Thanks a lot,

Alan