docs: def build_rrule(count=None, interval=None, bysecond=None, byminute=None,
                  byhour=None, byweekno=None, bymonthday=None, byyearday=None,
                  bymonth=None, until=None, bysetpos=None, wkst=None, byday=None,
                  freq=None):
      """
      Build rrule dictionary for vRecur class.
  
      :param count: int
      :param interval: int
      :param bysecond: int
      :param byminute: int
      :param byhour: int
      :param byweekno: int
      :param bymonthday: int
      :param byyearday: int
      :param bymonth: int
      :param until: datetime
      :param bysetpos: int
      :param wkst: str, two-letter weekday
      :param byday: weekday
      :param freq: str, frequency name ('WEEK', 'MONTH', etc)
      :return: dict
      """
      result = {}
  
      if count is not None:
          result['COUNT'] = count
  
      if interval is not None:
          result['INTERVAL'] = interval
  
      if bysecond is not None:
          result['BYSECOND'] = bysecond
  
      if byminute is not None:
          result['BYMINUTE'] = byminute
  
      if byhour is not None:
          result['BYHOUR'] = byhour
  
      if byweekno is not None:
          result['BYWEEKNO'] = byweekno
  
      if bymonthday is not None:
          result['BYMONTHDAY'] = bymonthday
  
      if byyearday is not None:
          result['BYYEARDAY'] = byyearday
  
      if bymonth is not None:
          result['BYMONTH'] = bymonth
  
      if until is not None:
          result['UNTIL'] = until
  
      if bysetpos is not None:
          result['BYSETPOS'] = bysetpos
  
      if wkst is not None:
          result['WKST'] = wkst
  
      if byday is not None:
          result['BYDAY'] = byday
  
      if freq is not None:
          if freq not in vRecur.frequencies:
              raise ValueError('Frequency value should be one of: {0}'
                               .format(vRecur.frequencies))
          result['FREQ'] = freq
  
      return result