VOunits draft

[Francois Ochsenbein]

Hi Steve,

This is the generic problem of introducing new units, and there
should be a generic way of solving it: the prefix rule applies only
when the unit is not recognized in a list of the accepted units. 
In other terms, since 'Pa' is a valid unit, no attempt would be 
made to try to understand it as a prefix + accepted unit -- 
therefore no nocessity of dealing with special cases, but just 
a standard interpretation.

--Francois

>
>On Tue 2009-05-26T20:56:50 +0100, Anita M. S. Richards hath writ:
>> I very much hope that we will do all of these - and Mpc and even Ypc....
>
>Take a close look at the FITS WCS paper 1 table.
>Along with all the true SI units it admits a number of astronomical
>units, in particular "a" for "annum", year.
>
>But there is an SI unit pascal, "Pa", which is thus indistinguishable
>from Pa, for "peta annum".
>
>As a result, the FITS WCS standard makes an explicit exception by
>asserting that Pa may not mean "peta annum".
>
>This is simply a "heads up" that to admit any units other than SI is
>to open the door to various sorts of special cases which have to be
>coded into all implementations of unit parsing code.
>
>--
>Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
>UCO/Lick Observatory        Natural Sciences II, Room 165    Lat  +36.99855
>University of California    Voice: +1 831 459 3046           Lng -122.06015
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Francois Ochsenbein    ------   Observatoire Astronomique de Strasbourg
   11, rue de l'Universite 67000 STRASBOURG  Phone: +33-(0)390 24 24 29
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