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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _granule_identifier_to_xml_name(granule_identifier):
 """ Very ugly way to convert the granule identifier. e.g. From Granule Identifier: S2A_OPER_MSI_L1C_TL_SGS__20150817T131818_A000792_T28QBG_N01.03 To Granule Metadata XML name: S2A_OPER_MTD_L1C_TL_SGS__20150817T131818_A000792_T28QBG.xml """

    # Replace "MSI" with "MTD".
    changed_item_type = re.sub("_MSI_", "_MTD_", granule_identifier)
    # Split string up by underscores.
    split_by_underscores = changed_item_type.split("_")
    del split_by_underscores[-1]
    cleaned = str()
    # Stitch string list together, adding the previously removed underscores.
    for i in split_by_underscores:
        cleaned += (i + "_")
    # Remove last underscore and append XML file extension.
    out_xml = cleaned[:-1] + ".xml"

    return out_xml