xml:space declaration

This is a request for clarification of the allowed declarations of
the xml:space attribute.

Section 2.10 says:

  When declared, it must be given as an enumerated type whose only
  possible values are "default" and "preserve".  For example:

  <!ATTLIST poem xml:space (default|preserve) 'preserve'>

The natural interpretation of this seems to me to be that it must be
declared as an enumeration, and that there must be precisely two
values in the enumeration, namely "default" and "preserve".

Others have interpreted it as meaning that it must be declared as
an enumeration, and that the only values allowed in that enumeration
are "default" and "preserve".

So in particular is it legal to declare it with only one of "default"
and "preserve" in the enumeration?

Note that the XHTML 1.0 DTDs declare it as:

  xml:space (preserve) #FIXED 'preserve'

and several parsers, including MSXML, report it as incorrect.

-- Richard