goxpath.go
func (xp XPathExec) Exec(t tree.Node, opts ...FuncOpts) (tree.Result, error) {
	o := &Opts{
		NS:    make(map[string]string),
		Funcs: make(map[xml.Name]tree.Wrap),
		Vars:  make(map[string]tree.Result),
	}
	for _, i := range opts {
		i(o)
	}
	return execxp.Exec(xp.n, t, o.NS, o.Funcs, o.Vars)
}
func (xp XPathExec) Exec(t tree.Node, opts ...FuncOpts) (tree.Result, error) {
	o := &Opts{
		NS:    make(map[string]string),
		Funcs: make(map[xml.Name]tree.Wrap),
		Vars:  make(map[string]tree.Result),
	}
	for _, i := range opts {
		i(o)
	}
	return execxp.Exec(xp.n, t, o.NS, o.Funcs, o.Vars)
}
//Exec executes the XPath expression, xp, against the tree, t, with the
//namespace mappings, ns, and returns the result as a stringer.