Looking for info on Pear concepts and classes

Hi I've been made volunteer to write down a paper about PEAR concepts and classes. I've read the coding guidelines and the error handling functions, generated the documentation of all the classes downloaded from the latest CVS repository. Then I found an article on phpbuilder http://www.phpbuilder.com/columns/sean20000712.php3 that somehow introduces the kind of things I am looking for. That article started to clarify me some concepts, but other remain in the darkness, for example about the 'factory' , what the 'destructor' , the installer, etc. Is there any other article you know about on the same, general style? There is something that I need to know also about the licensing scheme. As you download PEAR with your PHP distribution, what can you do with the classes present there? The question, in practice, is wether someone can think to develop and distribute some in-house project that fits into his PEAR tree, to guarantee compliance, unified install methods, docu and integration with other packages and PEAR itself. Can you do that, and with what licensing constraints? Thanks a lot. Giancarlo