Document-based apps/Alert sheet

FROM : Francis Derive
DATE : Fri Jun 23 18:11:42 2006

La liste,

When I click on the window top left close button because the document 
has been edited but not saved, an alert sheet is displayed to get a 
chance to save it : this is the material "provided for free" by the 
Cocoa framework.
How do I customize this sheet ?
This could be either to disable the Save... button ( because the 
document not having a "valid" content ), or acting on the 
implemention of the action of the Save... button so as to provide for 
the same content validation checking.

Merci.