code: def after(self, idx):
          
          if not isinstance(idx, datetime):
              raise TypeError("'%s' is not %s" % (idx, datetime))
          day = max(idx.date(), self._lo_limit)
          while day < self._hi_limit:
              if day < self._rd_cache.lo or day >= self._rd_cache.hi:
                  self._load(self._rd_cache, day)
              self._rd_cache.set_ptr(idx)
              if self._rd_cache.ptr < len(self._rd_cache.data):
                  return self._rd_cache.data[self._rd_cache.ptr]['idx']
              day = self._rd_cache.hi
          return None
docs: Return datetime of oldest existing data record whose
          datetime is >= idx.
  
          Might not even be in the same year! If no such record exists,
          return None.