There is some uncertainty about whether XSD 1.0 allows or forbids
the use of anySimpleType as a member type in constructing a union
or as an item type when constructing a list.

It can be (and has been) argued that Structures section 3.4.16, 
validation rule Derivation Valid (Restriction, Simple) rules it 
out in clause 3.1, which requires that the member types of
a union must have {variety} = list or {variety} = union.

But it has also been argued that there is material in Datatypes which
suggests that anySimpleType is indeed legal as a member of a union.

We may need clarification in the text of 1.0, either way.  And
some WG members would like agreement on whether anySimpleType can
be a member of a union in 1.0, before deciding what the rules
should be in 1.1 and closing bug 2306.