> No less ugly or off topic, but you could also do:
> 
> <!ELEMENT personalinfo   ssn>
> <!ELEMENT ssn  (0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9)+>
> <!ELEMENT 0  EMPTY>
> ...
> 
> to allow ssn numbers of variable lengths.

No, the first declaration is invalid (ssn should be in parentheses), and an
XML element name/type must match the Name production and thus cannot begin
with a number. Even if it were valid, this approach would mean his XML would
look like this:

<personalinfo>
  <ssn>
    <1/><1/><1/><2/><2/><3/><3/><3/><3/>
  </ssn>
</personalinfo>


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