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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 mergebydepth(args):
 """ %prog mergebydepth reads.bed genome.fasta Similar to mergeBed, but only returns regions beyond certain depth. """

    p = OptionParser(mergebydepth.__doc__)
    p.add_option("--mindepth", default=3, type="int",
                 help="Minimum depth required")
    opts, args = p.parse_args(args)

    if len(args) != 2:
        sys.exit(not p.print_help())

    bedfile, fastafile = args
    mindepth = opts.mindepth
    bedgraph = make_bedgraph(bedfile)

    bedgraphfiltered = bedgraph + ".d{0}".format(mindepth)
    if need_update(bedgraph, bedgraphfiltered):
        filter([bedgraph, "--minaccn={0}".format(mindepth),
                "--outfile={0}".format(bedgraphfiltered)])

    merged = bedgraphfiltered + ".merge.fasta"
    if need_update(bedgraphfiltered, merged):
        mergeBed(bedgraphfiltered, sorted=True)