In  http://lists.w3.org/Archives/Member/w3c-xml-query-wg/2009Feb/0018.html (member only) Zhen raised questions about the definition of "vacuous expression". The WG agreed to improve the definition; this bugzilla entry is being raised posthumously to capture the decision.

The definition will be changed to:

[Definition: Certain expressions are defined in this specification to be "vacuous expresions". These all have the characteristic that they can be determined statically to either return an empty sequence or raise an error.]
Some expressions are always vacuous; for instance, an empty parenthesized expression ( ) is a vacuous expression. Other expressions may be vacuous if one of their operands is vacuous; for instance, if both branches of a conditional expression are vacuous, the conditional expression is a vacuous expression.