docs: def fasta(self, key='vdj_nt', append_chain=True):
          '''
          Returns the sequence pair as a fasta string. If the Pair object contains
          both heavy and light chain sequences, both will be returned as a single string.
  
          By default, the fasta string contains the 'vdj_nt' sequence for each chain. To change,
          use the <key> option to select an alternate sequence.
  
          By default, the chain (heavy or light) will be appended to the sequence name:
  
          >MySequence_heavy
  
          To just use the pair name (which will result in duplicate sequence names for Pair objects
          with both heavy and light chains), set <append_chain> to False.
          '''
          fastas = []
          for s, chain in [(self.heavy, 'heavy'), (self.light, 'light')]:
              if s is not None:
                  c = '_{}'.format(chain) if append_chain else ''
                  fastas.append('>{}{}\n{}'.format(s['seq_id'], c, s[key]))
          return '\n'.join(fastas)