def overlap(args):
    """
    %prog overlap ctgfasta poolfasta

    Fish out the sequences in `poolfasta` that overlap with `ctgfasta`.
    Mix and combine using `minimus2`.
    """
    p = OptionParser(overlap.__doc__)
    opts, args = p.parse_args(args)

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

    ctgfasta, poolfasta = args
    prefix = ctgfasta.split(".")[0]
    rid = list(Fasta(ctgfasta).iterkeys())
    assert len(rid) == 1, "Use overlapbatch() to improve multi-FASTA file"

    rid = rid[0]
    splitctgfasta = ctgfasta.rsplit(".", 1)[0] + ".split.fasta"
    ctgfasta = run_gapsplit(infile=ctgfasta, outfile=splitctgfasta)

    # Run BLAST
    blastfile = ctgfasta + ".blast"
    run_megablast(infile=ctgfasta, outfile=blastfile, db=poolfasta)

    # Extract contigs and merge using minimus2
    closuredir = prefix + ".closure"
    closure = False
    if need_update(blastfile, closuredir):
        mkdir(closuredir, overwrite=True)
        closure = True

    if closure:
        idsfile = op.join(closuredir, prefix + ".ids")
        cmd = "cut -f2 {0} | sort -u".format(blastfile)
        sh(cmd, outfile=idsfile)

        idsfastafile = op.join(closuredir, prefix + ".ids.fasta")
        cmd = "faSomeRecords {0} {1} {2}".format(poolfasta, idsfile, idsfastafile)
        sh(cmd)

        # This step is a hack to weight the bases from original sequences more
        # than the pulled sequences, by literally adding another copy to be used
        # in consensus calls.
        redundantfastafile = op.join(closuredir, prefix + ".redundant.fasta")
        format([ctgfasta, redundantfastafile, "--prefix=RED."])

        mergedfastafile = op.join(closuredir, prefix + ".merged.fasta")
        cmd = "cat {0} {1} {2}".format(ctgfasta, redundantfastafile, idsfastafile)
        sh(cmd, outfile=mergedfastafile)

        afgfile = op.join(closuredir, prefix + ".afg")
        cmd = "toAmos -s {0} -o {1}".format(mergedfastafile, afgfile)
        sh(cmd)

        cwd = os.getcwd()
        os.chdir(closuredir)
        cmd = "minimus2 {0} -D REFCOUNT=0".format(prefix)
        cmd += " -D OVERLAP=100 -D MINID=98"
        sh(cmd)
        os.chdir(cwd)

    # Analyze output, make sure that:
    # + Get the singletons of the original set back
    # + Drop any contig that is comprised entirely of pulled set
    originalIDs = set(Fasta(ctgfasta).iterkeys())
    minimuscontig = op.join(closuredir, prefix + ".contig")
    c = ContigFile(minimuscontig)
    excludecontigs = set()
    for rec in c.iter_records():
        reads = set(x.id for x in rec.reads)
        if reads.isdisjoint(originalIDs):
            excludecontigs.add(rec.id)

    logging.debug("Exclude contigs: {0}".\
            format(", ".join(sorted(excludecontigs))))

    finalfasta = prefix + ".improved.fasta_"
    fw = open(finalfasta, "w")
    minimusfasta = op.join(closuredir, prefix + ".fasta")
    f = Fasta(minimusfasta)
    for id, rec in f.iteritems_ordered():
        if id in excludecontigs:
            continue
        SeqIO.write([rec], fw, "fasta")

    singletonfile = op.join(closuredir, prefix + ".singletons")
    singletons = set(x.strip() for x in open(singletonfile))
    leftovers = singletons & originalIDs

    logging.debug("Pull leftover singletons: {0}".\
            format(", ".join(sorted(leftovers))))

    f = Fasta(ctgfasta)
    for id, rec in f.iteritems_ordered():
        if id not in leftovers:
            continue
        SeqIO.write([rec], fw, "fasta")

    fw.close()

    fastafile = finalfasta
    finalfasta = fastafile.rstrip("_")
    format([fastafile, finalfasta, "--sequential", "--pad0=3",
        "--prefix={0}_".format(rid)])

    logging.debug("Improved FASTA written to `{0}`.".format(finalfasta))

    n50([ctgfasta])
    n50([finalfasta])

    errlog = "error.log"
    for f in (fastafile, blastfile, errlog):
        if op.exists(f):
            os.remove(f)