func Exec(n *parser.Node, t tree.Node, ns map[string]string, fns map[xml.Name]tree.Wrap, v map[string]tree.Result) (tree.Result, error) {
	f := xpFilt{
		t:         t,
		ns:        ns,
		ctx:       tree.NodeSet{t},
		fns:       fns,
		variables: v,
	}

	return exec(&f, n)
}
func Exec(n *parser.Node, t tree.Node, ns map[string]string, fns map[xml.Name]tree.Wrap, v map[string]tree.Result) (tree.Result, error) {
	f := xpFilt{
		t:         t,
		ns:        ns,
		ctx:       tree.NodeSet{t},
		fns:       fns,
		variables: v,
	}

	return exec(&f, n)
}
//Exec executes the XPath expression, xp, against the tree, t, with the
//namespace mappings, ns.