code: def get_residue_mapping(self):
          '''Returns a mapping between the sequences ONLY IF there are exactly two. This restriction makes the code much simpler.'''
          if len(self.sequence_ids) == 2:
              if not self.alignment_output:
                  self.align()
              assert(self.alignment_output)
              return self._create_residue_map(self._get_alignment_lines(), self.sequence_ids[1], self.sequence_ids[2])
          else:
              return Nonf get_residue_mapping(self):
          '''Returns a mapping between the sequences ONLY IF there are exactly two. This restriction makes the code much simpler.'''
          if len(self.sequence_ids) == 2:
              if not self.alignment_output:
                  self.align()
              assert(self.alignment_output)
              return self._create_residue_map(self._get_alignment_lines(), self.sequence_ids[1], self.sequence_ids[2])
          else:
              return None
docs: Returns a mapping between the sequences ONLY IF there are exactly two. This restriction makes the code much simpler.