/// <summary>
/// Declare a namespace for use by the query. This has the same
/// status as a namespace appearing within the query prolog (though
/// a declaration in the query prolog of the same prefix will take
/// precedence)
/// </summary>
/// <param name="prefix">The namespace prefix to be declared. Use
/// a zero-length string to declare the default namespace (that is, the
/// default namespace for elements and types).</param>
/// <param name="uri">The namespace URI. It is possible to specify
/// a zero-length string to "undeclare" a namespace.</param>
public void DeclareNamespace(String prefix, String uri)
{
env.declareNamespace(prefix, uri);
}