human
Complete the below
def PdbAtomIterator(handle):

    ''''Returns SeqRecord objects for each chain in a PDB file
    The sequences are derived from the 3D structure (ATOM records), not the
    SEQRES lines in the PDB file header.
    Unrecognised three letter amino acid codes (e.g. "CSD") from HETATM entries
    are converted to "X" in the sequence.
    In addition to information from the PDB header (which is the same for all
    records), the following chain specific information is placed in the
    annotation:
    record.annotations["residues"] = List of residue ID strings
    record.annotations["chain"] = Chain ID (typically A, B ,...)
    record.annotations["model"] = Model ID (typically zero)
    Where amino acids are missing from the structure, as indicated by residue
    numbering, the sequence is filled in with \'X\' characters to match the size
    of the missing region, and  None is included as the corresponding entry in
    the list record.annotations["residues"].
    This function uses the Bio.PDB module to do most of the hard work. The
    annotation information could be improved but this extra parsing should be
    done in parse_pdb_header, not this module.
    This gets called internally via Bio.SeqIO for the atom based interpretation
    of the PDB file format:
    >>> from Bio import SeqIO
    >>> for record in SeqIO.parse("PDB/1A8O.pdb", "pdb-atom"):
    ...     print("Record id %s, chain %s" % (record.id, record.annotations["chain"]))
    Record id 1A8O:A, chain A
    Equivalently,
    >>> with open("PDB/1A8O.pdb") as handle:
    ...     for record in PdbAtomIterator(handle):
    ...         print("Record id %s, chain %s" % (record.id, record.annotations["chain"]))
    Record id 1A8O:A, chain A'
    '''

=====
Write me a the Python program you requested?
assistant
#Here is the Python program you requested.
    from Bio.PDB import PDBParser
    from Bio.SeqUtils import seq1
    def restype(residue):
        "Return    a    residue's    type    as    a    one-letter    code.\n\n                                Non-standard    residues    (e.g.    CSD,    ANP)    are    returned    as    'X'.\n                                "
        return seq1(residue.resname, custom_map=protein_letters_3to1)
    from Bio.File import UndoHandle
    undo_handle = UndoHandle(handle)
    firstline = undo_handle.peekline()
    if firstline.startswith('HEADER'):
        pdb_id = firstline[62:66]
    else:
        warnings.warn(("First    line    is    not    a    'HEADER';    can't    determine    PDB    ID.    Line:    %r" % firstline), BiopythonWarning)
        pdb_id = '????'
    struct = PDBParser().get_structure(pdb_id, undo_handle)
    model = struct[0]
    for (chn_id, chain) in sorted(model.child_dict.items()):
        residues = [res for res in chain.get_unpacked_list() if (seq1(res.get_resname().upper(), custom_map=protein_letters_3to1) != 'X')]
        if (not residues):
            continue
        gaps = []
        rnumbers = [r.id[1] for r in residues]
        for (i, rnum) in enumerate(rnumbers[:(-1)]):
            if (rnumbers[(i + 1)] != (rnum + 1)):
                gaps.append(((i + 1), rnum, rnumbers[(i + 1)]))
        if gaps:
            res_out = []
            prev_idx = 0
            for (i, pregap, postgap) in gaps:
                if (postgap > pregap):
                    gapsize = ((postgap - pregap) - 1)
                    res_out.extend((restype(x) for x in residues[prev_idx:i]))
                    prev_idx = i
                    res_out.append(('X' * gapsize))
                else:
                    warnings.warn('Ignoring    out-of-order    residues    after    a    gap', BiopythonWarning)
                    res_out.extend((restype(x) for x in residues[prev_idx:i]))
                    break
            else:
                res_out.extend((restype(x) for x in residues[prev_idx:]))
        else:
...