The <copyright> element is able to hold several <year>
elements, which is good for simple enumeration of
years, but not for ranges, like "(c) 1990-2001", where
it would be tedious and inaccurate to specify each year
of the interval.
I suggest the following extension:
<!ELEMENT Copyright - - ((Year|YearRange)+, Holder*)>
<!ELEMENT YearRange - - (Year,Year)>