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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 _check_jointcaller(data):
 """Ensure specified jointcaller is valid. """

    allowed = set(joint.get_callers() + [None, False])
    cs = data["algorithm"].get("jointcaller", [])
    if not isinstance(cs, (tuple, list)):
        cs = [cs]
    problem = [x for x in cs if x not in allowed]
    if len(problem) > 0:
        raise ValueError("Unexpected algorithm 'jointcaller' parameter: %s\n"
                         "Supported options: %s\n" % (problem, sorted(list(allowed), key=lambda x: x or "")))