Convert MRC data format to MARC XML.

    Args:
        mrc (str): MRC as string.

    Returns:
        str: XML with MARC.

def mrc_to_marc(mrc):
    """
    Convert MRC data format to MARC XML.

    Args:
        mrc (str): MRC as string.

    Returns:
        str: XML with MARC.
    """
    # ignore blank lines
    lines = [
        line
        for line in mrc.splitlines()
        if line.strip()
    ]

    def split_to_parts(lines):
        for line in lines:
            first_part, second_part = line.split(" L ", 1)

            yield line, first_part, second_part.lstrip()

    control_lines = []
    data_lines = []
    for line, first_part, second_part in split_to_parts(lines):
        if second_part.startswith("$"):
            data_lines.append(line)
        else:
            control_lines.append(line)

    # convert controlfield lines
    record = MARCXMLRecord()
    record.oai_marc = True
    for line, descr, content in split_to_parts(control_lines):
        record.controlfields[descr.strip()[:3]] = content

    def get_subfield_dict(line):
        fields = (
            (field[0], field[1:])
            for field in line.split("$$")[1:]
        )

        fields_dict = defaultdict(list)
        for key, val in fields:
            fields_dict[key].append(val)

        return fields_dict

    # convert datafield lines
    for line, descr, content_line in split_to_parts(data_lines):
        name = descr[:3]
        i1 = descr[3]
        i2 = descr[4]

        record.add_data_field(
            name,
            i1,
            i2,
            get_subfield_dict(content_line)
        )

    return record.to_XML()