def ld_expand(df, ld_beds):
    """
    Expand a set of SNVs into all SNVs with LD >= 0.8 and return a BedTool of
    the expanded SNPs.
    
    Parameters
    ----------
    df : pandas.DataFrame
        Pandas dataframe with SNVs. The index is of the form chrom:pos where pos
        is the one-based position of the SNV. The columns are chrom, start, end.
        chrom, start, end make a zero-based bed file with the SNV coordinates.

    ld_beds : dict
        Dict whose keys are chromosomes and whose values are filenames of
        tabixed LD bed files. The LD bed files should be formatted like this:
            chr1    14463   14464   14464:51479:0.254183
        where the the first three columns indicate the zero-based coordinates of
        a SNV and the the fourth column has the one-based coordinate of that
        SNV, the one-based coordinate of another SNV on the same chromosome, and
        the LD between these SNVs (all separated by colons).

    Returns
    -------
    bt : pybedtools.BedTool
        BedTool with input SNVs and SNVs they are in LD with.
        indepdent SNVs.
    """
    import pybedtools as pbt
    import tabix
    out_snps = []
    for chrom in ld_beds.keys():
        t = tabix.open(ld_beds[chrom])
        tdf = df[df['chrom'].astype(str) == chrom]
        for ind in tdf.index:
            p = tdf.ix[ind, 'end']
            out_snps.append('{}\t{}\t{}\t{}\n'.format(chrom, p - 1, p, ind))
            try:
                r = t.query('{}'.format(chrom), p - 1, p)
                while True:
                    try:
                        n = r.next()
                        p1, p2, r2 = n[-1].split(':')
                        if float(r2) >= 0.8:
                            out_snps.append('{}\t{}\t{}\t{}\n'.format(n[0],
                                int(p2) - 1, int(p2), ind))
                    except StopIteration:
                        break
            except tabix.TabixError:
                continue
    bt = pbt.BedTool(''.join(out_snps), from_string=True)
    bt = bt.sort()
    return bt
