<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def liftover(args):
 """ %prog liftover lobstr_v3.0.2_hg38_ref.bed hg38.upper.fa LiftOver CODIS/Y-STR markers. """

    p = OptionParser(liftover.__doc__)
    p.add_option("--checkvalid", default=False, action="store_true",
                help="Check minscore, period and length")
    opts, args = p.parse_args(args)

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

    refbed, fastafile = args
    genome = pyfasta.Fasta(fastafile)
    edits = []
    fp = open(refbed)
    for i, row in enumerate(fp):
        s = STRLine(row)
        seq = genome[s.seqid][s.start - 1: s.end].upper()
        s.motif = get_motif(seq, len(s.motif))
        s.fix_counts(seq)
        if opts.checkvalid and not s.is_valid():
            continue
        edits.append(s)
        if i % 10000 == 0:
            print(i, "lines read", file=sys.stderr)

    edits = natsorted(edits, key=lambda x: (x.seqid, x.start))
    for e in edits:
        print(str(e))