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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 _calculate_sv_bins_gatk(data, cnv_group, size_calc_fn):
 """Calculate structural variant bins using GATK4 CNV callers region or even intervals approach. """

    if dd.get_background_cnv_reference(data, "gatk-cnv"):
        target_bed = gatkcnv.pon_to_bed(dd.get_background_cnv_reference(data, "gatk-cnv"), cnv_group.work_dir, data)
    else:
        target_bed = gatkcnv.prepare_intervals(data, cnv_group.region_file, cnv_group.work_dir)
    gc_annotated_tsv = gatkcnv.annotate_intervals(target_bed, data)
    return target_bed, None, gc_annotated_tsv