the reference chromosome. greedy return the first chromosome in exon array

    :return: chromosome
    :rtype: string

def chr(self):
    """the reference chromosome. greedy return the first chromosome in exon array

    :return: chromosome
    :rtype: string
    """
    if len(self.exons)==0: 
      sys.stderr.write("WARNING can't return chromsome with nothing here\n")
      return None
    return self._rngs[0].chr