  Two natural decision problems regarding the XML query language XQuery are
well-definedness and semantic type-checking. We study these problems in the
setting of a relational fragment of XQuery. We show that well-definedness and
semantic type-checking are undecidable, even in the positive-existential case.
Nevertheless, for a ``pure'' variant of XQuery, in which no identification is
made between an item and the singleton containing that item, the problems
become decidable. We also consider the analogous problems in the setting of the
nested relational calculus.
