System Category List Update

["Eric Arseneau" <eat@huv.com>]

Can anybody explain to me how the system category list in a browser under
morphic is updated.  I see no dependencies, no chnged: signallin of any
kind, yet it happens automagically.

Remove a system category from one browser and all the others update.  How is
this accomplished ?

I've been looking and can't find anything.  This may be yet another sign of
my own stupidity.  If that is the case then this gives the chance of some
other brighter soul to illuminate me ;-)