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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 standardize_cnv_reference(data):
 """Standardize cnv_reference background to support multiple callers. """

    out = tz.get_in(["config", "algorithm", "background", "cnv_reference"], data, {})
    cur_callers = set(data["config"]["algorithm"].get("svcaller")) & _CNV_REFERENCE
    if isinstance(out, six.string_types):
        if not len(cur_callers) == 1:
            raise ValueError("Multiple CNV callers and single background reference for %s: %s" %
                                data["description"], list(cur_callers))
        else:
            out = {cur_callers.pop(): out}
    return out