Mr. Lapp has discovered one of the well-known shortcomings of SGML,
inherited by XML; namely, the typing and constraint mechanisms supplied
by DTDs are well-known to be insufficiently rich to allow their use
for purposes which we have come to expect of database schemas.
More obviously, if, in Mr. Lapp's example, I wanted to give prices
for the books, I might want to be able to say that this has to be
a number, with 2 digits right of the decimal point. SGML doesn't
help you here either.
Yes; we need a new and richer form of schema. No boldness is
required. -Tim