Valid use of the Where clause

I have been reading the specs on 1.0 working draft for XQuery regarding the
where clause.  It does not seem to support the following syntax:
Let $a := input()/person/lastname
Where $a = "miller"<?xml:namespace prefix = o ns =
"urn:schemas-microsoft-com:office:office" />
Return $a
 
Notice in the where clause there is no further navigation down the tree
because this query wants all documents with lastname = to miller.  All spec
examples do not show this usage of the where clause.  Is the above query
syntactically correct based on the 1.0 standards?
many thanks in advance
Shane