  XML is of great importance in information storage and retrieval because of
its recent emergence as a standard for data representation and interchange on
the Internet. However XML provides little semantic content and as a result
several papers have addressed the topic of how to improve the semantic
expressiveness of XML. Among the most important of these approaches has been
that of defining integrity constraints in XML. In a companion paper we defined
strong functional dependencies in XML(XFDs). We also presented a set of axioms
for reasoning about the implication of XFDs and showed that the axiom system is
sound for arbitrary XFDs. In this paper we prove that the axioms are also
complete for unary XFDs (XFDs with a single path on the l.h.s.). The second
contribution of the paper is to prove that the implication problem for unary
XFDs is decidable and to provide a linear time algorithm for it.
