def _check_aligner(item):
    """Ensure specified aligner is valid choice.
    """
    allowed = set(list(alignment.TOOLS.keys()) + [None, False])
    if item["algorithm"].get("aligner") not in allowed:
        raise ValueError("Unexpected algorithm 'aligner' parameter: %s\n"
                         "Supported options: %s\n" %
                         (item["algorithm"].get("aligner"), sorted(list(allowed))))
Ensure specified aligner is valid choice.