Set REJECT in VCF line, or add it if there is something else.

def _set_reject(line):
    """Set REJECT in VCF line, or add it if there is something else."""
    if line.startswith("#"):
        return line
    parts = line.split("\t")
    if parts[6] == "PASS":
        parts[6] = "REJECT"
    else:
        parts[6] += ";REJECT"
    return "\t".join(parts)