def flip_mat_pat(snps):
    """Take a dictionary of SNPs and flip all mat and pat sites.

    Note: This changes the original dictionary and does not make a copy.
    """
    for chr, poss in snps.items():
        for pos, snp in poss.items():
            mat = snp.mat
            pat = snp.pat
            snp.mat = pat
            snp.pat = mat
    return snps