This module implements draft 0.4 of John Cowan's XML Catalog (formerly
known as XCatalog) proposal (<http://www.ccil.org/~cowan/XML/XCatalog.html>).
Catalogs may be written in either SOCAT or XML syntax (see the proposal
for syntax details); XML::Catalog will assume SOCAT syntax if the catalog
is not in well-formed XML syntax.