function markCurrent(index) {
        // Remove previous highlight first
        if ($markedTickmark) {
            $markedTickmark.removeClass("tickmark-current");
            $markedTickmark = null;
        }
        if (index !== -1) {
            $markedTickmark = $(".tickmark-track > .tickmark", editor.getRootElement()).eq(index).addClass("tickmark-current");
        }
    }
@param {number} index Either -1, or an index into the array passed to addTickmarks()