code: def different_random_nt(self,nt):
      global nts
      
      return self._random.choice([x for x in nts if x != nt.upper()])
docs: generate a random nucleotide change. uniform random.  will never return itself
  
      :param nt: current nucleotide
      :type nt: char
      :return: new nucleotide
      :rtype: char