docs: def _set_seq(self,sequence,start=0):
      """Set the sequence from a start position for the length of the sequence"""
      if start+len(sequence) > self._slen: 
        sys.stderr.write("Error not long enough to add\n")
        sys.exit()
      z = 0
      for i in xrange(start, start+len(sequence)):
        self._set_nt(sequence[z],i)
        z+=1