Basic units in XML (Was: Re: [QUANTITY] Units)

[Jonathan McDowell]

Brian,
 My take is that the worst thing to do is specify a particular basic set.
I may want to revert to my theorist childhood and only have one basic unit,
the GeV, which everything else is expressed in turns of. Or I may prefer
(Msun, AU, yr) instead of (kg, m, s). So the trick is to *support* the concept
of a basic set, not to pick what that basic set is.
  Jonathan