docs: def size(self, pairs_only=False):
          '''
          Calculate the size of the lineage.
  
          Inputs (optional)
          -----------------
          pairs_only: count only paired sequences
  
          Returns
          -------
          Lineage size (int)
          '''
          if pairs_only:
              return len(self.just_pairs)
          else:
              return len(self.heavies)