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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 plot_multiple_regions_coverage(samples, out_file, data, region_bed=None, stem_bed=None):
 """ given a list of bcbio samples and a bed file or BedTool of regions, makes a plot of the coverage in the regions for the set of samples if given a bed file or BedTool of locations in stem_bed with a label, plots lollipops at those locations """

    mpl.use('Agg', force=True)
    PAD = 100
    if file_exists(out_file):
        return out_file
    in_bams = [dd.get_align_bam(x) for x in samples]
    samplenames = [dd.get_sample_name(x) for x in samples]
    if isinstance(region_bed, six.string_types):
        region_bed = pybedtools.BedTool(region_bed)
    if isinstance(stem_bed, six.string_types):
        stem_bed = pybedtools.BedTool(stem_bed)
    if stem_bed is not None:  # tabix indexed bedtools eval to false
        stem_bed = stem_bed.tabix()
    plt.clf()
    plt.cla()
    with file_transaction(out_file) as tx_out_file:
        with backend_pdf.PdfPages(tx_out_file) as pdf_out:
            sns.despine()
            for line in region_bed:
                for chrom, start, end in _split_regions(line.chrom, max(line.start - PAD, 0),
                                                        line.end + PAD):
                    df = _combine_regional_coverage(in_bams, samplenames, chrom,
                                                    start, end, os.path.dirname(tx_out_file), data)
                    plot = sns.tsplot(df, time="position", unit="chrom",
                                      value="coverage", condition="sample")
                    if stem_bed is not None:  # tabix indexed bedtools eval to false
                        interval = pybedtools.Interval(chrom, start, end)
                        _add_stems_to_plot(interval, stem_bed, samples, plot)
                    plt.title("{chrom}:{start}-{end}".format(**locals()))
                    pdf_out.savefig(plot.get_figure())
                    plt.close()
    return out_file