hi skunk(ette)s

I came across this interesting XML document that maps into a Sudoku puzzle,

<snip> ....
Examine the XML document attached that represents a Sudoku Problem. You  are required to write the DTD associated with this XML document so that it can be validated. State clearly any assumptions you make.
</snip>

happy coding  :-)

P.S :  This is  a 3rd year B.Sc  Software Engineering exam question