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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 validate_aggregation(agg):
 """Validate an aggregation for use in Vega-Lite. Translate agg to one of the following supported named aggregations: ['mean', 'sum', 'median', 'min', 'max', 'count'] Parameters agg : string or callable A string Supported reductions are ['mean', 'sum', 'median', 'min', 'max', 'count']. If agg is a numpy function, the return value is the string representation. If agg is unrecognized, raise a ValueError """

    if agg is None:
        return agg
    supported_aggs = ['mean', 'sum', 'median', 'min', 'max', 'count']
    numpy_aggs = {getattr(np, a): a
                  for a in ['mean', 'sum', 'median', 'min', 'max']}
    builtin_aggs = {min: 'min', max: 'max', sum: 'sum'}

    agg = numpy_aggs.get(agg, agg)
    agg = builtin_aggs.get(agg, agg)

    if agg not in supported_aggs:
        raise ValueError("Unrecognized Vega-Lite aggregation: {0}".format(agg))

    return agg