docs: function parse_ExternSheet(blob, length, opts) {
  	if(opts.biff < 8) return parse_ShortXLUnicodeString(blob, length, opts);
  	var o = [], target = blob.l + length, len = blob.read_shift(2);
  	while(len-- !== 0) o.push(parse_XTI(blob, 6));
  		// [iSupBook, itabFirst, itabLast];
  	var oo = [];
  	return o;
  }