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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 _total_jxn_counts(fns):
 """Count the total unique coverage junction for junctions in a set of SJ.out.tab files."""

    df = pd.read_table(fns[0], header=None, names=COLUMN_NAMES)
    df.index = (df.chrom + ':' + df.start.astype(int).astype(str) + '-' +
                df.end.astype(int).astype(str))
    counts = df.unique_junction_reads
    for fn in fns[1:]:
        df = pd.read_table(fn, header=None, names=COLUMN_NAMES)
        df.index = (df.chrom + ':' + df.start.astype(int).astype(str) + '-' +
                    df.end.astype(int).astype(str))
        counts = counts.add(df.unique_junction_reads, fill_value=0)
    return counts