func (t *Template) associate(new *Template, tree *parse.Tree) bool {
	if new.common != t.common {
		panic("internal error: associate not common")
	}
	if old := t.tmpl[new.name]; old != nil && parse.IsEmptyTree(tree.Root) && old.Tree != nil {
		// If a template by that name exists,
		// don't replace it with an empty template.
		return false
	}
	t.tmpl[new.name] = new
	return true