code: def add_germline_variants(self, germline_nucs, coding_pos):
          
          if len(germline_nucs) != len(coding_pos):
              raise ValueError('Each germline nucleotide should have a coding position')
  
          es = list(self.exon_seq)
          for i in range(len(germline_nucs)):
              gl_nuc, cpos = germline_nucs[i].upper(), coding_pos[i]
              if not utils.is_valid_nuc(gl_nuc):
                  raise ValueError('{0} is not a valid nucleotide'.format(gl_nuc))
              if cpos >= 0:
                  es[cpos] = gl_nuc
          self.exon_seq = ''.join(es)
docs: Add potential germline variants into the nucleotide sequence.
  
          Sequenced individuals may potentially have a SNP at a somatic mutation position.
          Therefore they may differ from the reference genome. This method updates the gene
          germline gene sequence to match the actual individual.
  
          Parameters
          ----------
          germline_nucs : list of str
              list of DNA nucleotides containing the germline letter
          coding_pos : int
              0-based nucleotide position in coding sequence
  
          NOTE: the self.exon_seq attribute is updated, no return value