code: def datetime_exists(dt, tz=None):
      
      if tz is None:
          if dt.tzinfo is None:
              raise ValueError('Datetime is naive and no time zone provided.')
          tz = dt.tzinfo
  
      dt = dt.replace(tzinfo=None)
  
      # This is essentially a test of whether or not the datetime can survive
      # a round trip to UTC.
      dt_rt = dt.replace(tzinfo=tz).astimezone(tzutc()).astimezone(tz)
      dt_rt = dt_rt.replace(tzinfo=None)
  
      return dt == dt_rt
docs: Given a datetime and a time zone, determine whether or not a given datetime
      would fall in a gap.
  
      :param dt:
          A :class:`datetime.datetime` (whose time zone will be ignored if ``tz``
          is provided.)
  
      :param tz:
          A :class:`datetime.tzinfo` with support for the ``fold`` attribute. If
          ``None`` or not provided, the datetime's own time zone will be used.
  
      :return:
          Returns a boolean value whether or not the "wall time" exists in ``tz``.