The Prob class

[Trevor Daniels]

I'm trying to complete the 'prob' entry in the Technical glossary. 
> From a position of ignorance I've come up with this:

"PRoperty OBjects, or @strong{probs} for short, are instances of
the @code{Prob} class, a simple base class for objects which have
mutable and immutable property alists.  The @code{Music} class
derives from @code{Prob}.  Instances of the @code{Prob} class are
also created to hold the formatted content of system grobs and
titling blocks during page layout."

Is this correct, accurate, complete?

As an aside, why does the Grob class not derive from Prob?

Trevor