code: function makePartSpan(value, doc) {
    var text = value;
    if (value.nodeType == 3) text = value.nodeValue;
    else value = doc.createTextNode(text);
  
    var span = doc.createElement("SPAN");
    span.isPart = true;
    span.appendChild(value);
    span.currentText = text;
    return span;
  }
docs: Create a SPAN node with the expected properties for document part spans.