Input to Best Practice 18: Assign unique identifiers to text items when possible

Hi all,

some input to BP 18:

We should align this formulation
"Your DTD or schema should provide xml:id (or an equivalent mechanism)"
 with BP 9, e.g. like:
"Your DTD or schema should provide xml:id (or a different attribute to 
be of type ID)"

The "Why do this" section of BP 9 contains a lot of material relevant 
for BP 18 as well. I'd propose to have  a reference from the "Why do 
this"  section of BP 9 to the "Why do this" section of BP 9.

Felix