function _getTagIDAtDocumentPos(editor, pos, markCache) {
        var match = _getMarkerAtDocumentPos(editor, pos, false, markCache);

        return (match) ? match.tagID : -1;
    }
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