docs: function write_obj_str(factory) {
  	return function write_str(wb, o) {
  		var idx = 0;
  		for(var i=0;i<wb.SheetNames.length;++i) if(wb.SheetNames[i] == o.sheet) idx=i;
  		if(idx == 0 && !!o.sheet && wb.SheetNames[0] != o.sheet) throw new Error("Sheet not found: " + o.sheet);
  		return factory.from_sheet(wb.Sheets[wb.SheetNames[idx]], o, wb);
  	};
  }