toNode() {
    var e = document.createElementNS("http://www.w3.org/1998/Math/MathML", this.type);
    for (var t in this.attributes)
      Object.prototype.hasOwnProperty.call(this.attributes, t) && e.setAttribute(t, this.attributes[t]);
    this.classes.length > 0 && (e.className = on(this.classes));
    for (var r = 0; r < this.children.length; r++)
      e.appendChild(this.children[r].toNode());
    return e;
  }
Converts the math node into a string, similar to innerText, but escaped.