handling metadata with multiple values

[Roy Williams]

> Naive question: What's wrong with the following syntax
>
> <UCDList>
>          <x1/>
>          <x2/>
> </UCDList>

Three problems:

(1) the schema for such a document would be impossibly large -- with an
extra 1500 or so elements defined.

(2) many people like build a parser for an XML document using a "binding
tool". This means making a class for each type of UCD with get and set
methods. In the case above, there would be at least
    x1.get(), x1.set(), x1.new()
    x2.get(), x2.set(), x2.new()
If each UCD were an element, there would an extra 1500 classes in the code!

(3) The UCD object is not so simple any more. In UCD version 2, there are
multiple words, for example "phot.flux.radio.1500-3000GHz, error, max"