[xsl] option and sequence element test

[ac <ac@xxxxxxxxxxxxx>]

Hi,

From what I can gather, short of using a schema, it is not possible to 
have an element check like
element(abc | def, type) or
element(abc | def)
ensuring that only abc or def elements are valid, for example in
<xsl:function name="f:elem" as="element(abc | def)?">
Could it be considered?

and even
<xsl:function name="f:elem" as="element((abc, def), type)*">
for returning sequences (e.g. tuples) of "abc" and "def", for example,  
as in
<abc>1</abc><def>1</def>
<abc>2</abc><def>2</def>
... or
<last>Skywalker</last><first>Anakin</first>
<last>Skywalker</last><first>Luke</first>

Thank you.

Regards,
ac