def _create_ca_file(anchor_list, filename):
    """
    Concatenate all the certificates (PEM format for the export) in
    'anchor_list' and write the result to file 'filename'. On success
    'filename' is returned, None otherwise.

    If you are used to OpenSSL tools, this function builds a CAfile
    that can be used for certificate and CRL check.
    """
    try:
        with open(filename, "w") as f:
            for a in anchor_list:
                s = a.output(fmt="PEM")
                f.write(s)
    except IOError:
        return None
    return filename