XML schema draft comments: 4.1.2.1: what if both base and simpleType?

Regarding the draft at
http://www.w3.org/TR/2004/PER-xmlschema-2-20040318/:

Section 4.1.2.1 says:

   {base type definition}
     The Simple Type Definition component resolved to by the actual
     value of the base [attribute] or the <simpleType> [children],
     whichever is present.

The wording doesn't address what happens if both the base attribute
and a simpleType child element are present or if neither is present,
and there doesn't seem to be any wording (at least not in 4.1.2.1)
that says that those cases are illegal.

Daniel