docs: def count_residues(self, record, pdb_record):
          '''Count the number of residues in the chains for the case.'''
          mutations = self.get_record_mutations(record)
          pdb_chains = set([m['Chain'] for m in mutations])
          assert(len(pdb_chains) == 1) # we expect monomeric cases
          pdb_chain = pdb_chains.pop()
          return len(pdb_record.get('Chains', {}).get(pdb_chain, {}).get('Sequence', ''))