code: def subseq(self, start_offset=0, end_offset=None):
          
          from sebastian.core import DURATION_64
  
          def subseq_iter(start_offset, end_offset):
              cur_offset = 0
              for point in self._elements:
                  try:
                      cur_offset += point[DURATION_64]
                  except KeyError:
                      raise ValueError("HSeq.subseq requires all points to have a %s attribute" % DURATION_64)
                  #Skip until start
                  if cur_offset < start_offset:
                      continue
  
                  #Yield points start_offset <=  point < end_offset
                  if end_offset is None or cur_offset < end_offset:
                      yield point
                  else:
                      raise StopIteration
          return HSeq(subseq_iter(start_offset, end_offset))
docs: Return a subset of the sequence
          starting at start_offset (defaulting to the beginning)
          ending at end_offset (None representing the end, whih is the default)
          Raises ValueError if duration_64 is missing on any element