docs: def get_sequence(self,chr=None,start=None,end=None,dir=None,rng=None):
      """get a sequence
  
      :param chr:
      :param start:
      :param end:
      :param dir: charcter +/-
      :parma rng:
      :type chr: string
      :type start: int
      :type end: int
      :type dir: char
      :type rng: GenomicRange
      :return: sequence 
      :rtype: string
      """
      if rng: 
        chr = rng.chr
        start = rng.start
        end = rng.end
        dir = rng.direction
      if not start: start = 1
      if not end: end = self.fai[chr]['length']
      if not dir: dir = '+'
      if dir == '-':
        return sequence.Sequence.rc(self._seqs[chr][start-1:end])
      return self._seqs[chr][start-1:end]