Pivot function Linq

public static Dictionary<TFirstKey, Dictionary<TSecondKey, TValue>> Pivot<TSource, TFirstKey, TSecondKey, TValue>(this IEnumerable<TSource> source, Func<TSource, TFirstKey> firstKeySelector, Func<TSource, TSecondKey> secondKeySelector, Func<IEnumerable<TSource>, TValue> aggregate)
{
    var retVal = new Dictionary<TFirstKey, Dictionary<TSecondKey, TValue>>();

    var l = source.ToLookup(firstKeySelector);
    foreach (var item in l)
    {
        var dict = new Dictionary<TSecondKey, TValue>();
        retVal.Add(item.Key, dict);
        var subdict = item.ToLookup(secondKeySelector);
        foreach (var subitem in subdict)
        {
            dict.Add(subitem.Key, aggregate(subitem));
        }
    }

    return retVal;
}