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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 _get_nblock_regions(in_file, min_n_size, ref_regions):
 """Retrieve coordinates of regions in reference genome with no mapping. These are potential breakpoints for parallelizing analysis. """

    out_lines = []
    called_contigs = set([])
    with utils.open_gzipsafe(in_file) as in_handle:
        for line in in_handle:
            contig, start, end, ctype = line.rstrip().split()
            called_contigs.add(contig)
            if (ctype in ["REF_N", "NO_COVERAGE", "EXCESSIVE_COVERAGE", "LOW_COVERAGE"] and
                  int(end) - int(start) > min_n_size):
                out_lines.append("%s\t%s\t%s\n" % (contig, start, end))
    for refr in ref_regions:
        if refr.chrom not in called_contigs:
            out_lines.append("%s\t%s\t%s\n" % (refr.chrom, 0, refr.stop))
    return pybedtools.BedTool("\n".join(out_lines), from_string=True)