code: func builtinWeekOfYear(args []types.Datum, ctx context.Context) (types.Datum, error) {
  	// WeekOfYear is equivalent to to Week(date, 3)
  	d := types.Datum{}
  	d.SetInt64(3)
  	return builtinWeek([]types.Datum{args[0], d}, ctx)
  }
docs: // See http://dev.mysql.com/doc/refman/5.7/en/date-and-time-functions.html#function_weekofyear