code: function _getTagIDAtDocumentPos(editor, pos, markCache) {
          var match = _getMarkerAtDocumentPos(editor, pos, false, markCache);
  
          return (match) ? match.tagID : -1;
      }
docs: Get the instrumented tagID at the specified position. Returns -1 if
  there are no instrumented tags at the location.
  The _markText() function must be called before calling this function.
  
  NOTE: This function is "private" for now (has a leading underscore), since
  the API is likely to change in the future.
  
  @param {Editor} editor The editor to scan.
  @param {{line: number, ch: number}} pos The position to find the DOM marker for.
  @param {Object=} markCache An optional cache to look up positions of existing
  markers. (This avoids calling the find() operation on marks multiple times,
  which is expensive.)
  @return {number} tagID at the specified position, or -1 if there is no tag