code: def _fetch_bounded_seq(self, var, start, end, window_size, boundary):
          
          var_len = end - start - window_size
  
          start = start if start >= boundary[0] else boundary[0]
          end = end if end <= boundary[1] else boundary[1]
          if start >= end:
              return ""
  
          seq = self.hdp.get_seq(var.ac, start, end)
  
          if len(seq) < end - start and len(seq) < var_len:
              raise HGVSInvalidVariantError(
                  "Variant span is outside sequence bounds ({var})".format(var=var))
  
          return seq
docs: Fetch reference sequence from hgvs data provider.
  
          The start position is 0 and the interval is half open