code: function parse_ExternSheet(blob, length, opts) {
  	if(opts.biff < 8) return parse_BIFF5ExternSheet(blob, length, opts);
  	var o = [], target = blob.l + length, len = blob.read_shift(opts.biff > 8 ? 4 : 2);
  	while(len-- !== 0) o.push(parse_XTI(blob, opts.biff > 8 ? 12 : 6, opts));
  		// [iSupBook, itabFirst, itabLast];
  	if(blob.l != target) throw new Error("Bad ExternSheet: " + blob.l + " != " + target);
  	return o;
  }
docs: /* [MS-XLS] 2.4.106 TODO: verify filename encoding