code: def replace_seqres(self, pdb, update_atoms = True):
      newpdb = PDB()
      inserted_seqres = False
      entries_before_seqres = set(["HEADER", "OBSLTE", "TITLE",  "CAVEAT", "COMPND", "SOURCE", 
                                   "KEYWDS", "EXPDTA", "AUTHOR", "REVDAT", "SPRSDE", "JRNL", 
                                   "REMARK", "DBREF",  "SEQADV"])
      mutated_resids = {}
      if update_atoms:
        old_seqs = ChainSequences()
        chainresnums = old_seqs.parse_atoms(pdb)
        assert self.keys() == old_seqs.keys()
        for chain in self.keys():
          assert len(self[chain]) == len(old_seqs[chain])
          for i in xrange(len(self[chain])):
            if self[chain][i] != old_seqs[chain][i]:
              resid = chain + chainresnums[chain][i]
              mutated_resids[resid] = self[chain][i]
      for line in pdb.lines:
        entry = line[0:6]
        if (not inserted_seqres) and entry not in entries_before_seqres:
          inserted_seqres = True
          newpdb.lines += self.seqres_lines()
        if update_atoms and entry == "ATOM  ":
          resid = line[21:27]
          atom = line[12:16].strip()
          if not mutated_resids.has_key(resid):
            newpdb.lines += [line]
          else:
            newpdb.lines += [line[:17] + mutated_resids[resid] + line[20:]]
        elif entry != "SEQRES":
          newpdb.lines += [line]
      if update_atoms:
        newpdb.remove_nonbackbone_atoms(mutated_resids.keys())
      return newpdb