_insertCursor(el, which) {
    this._writeImpl(`<span id="${which}"/>`);

    const cursor = this.doc.getElementById(which);

    if (cursor) {
      cursor.parentNode.replaceChild(el, cursor);
    }
  }
Append a span to the stream. That span will act as a cursor
(i.e. insertion point) for the element.

@param {Object} el The element
@param {string} which The type of proxy element