/// <summary>
/// See also https://msdn.microsoft.com/en-us/library/bb549393(v=vs.110).aspx
/// </summary>
/// <typeparam name="K"></typeparam>
/// <typeparam name="T"></typeparam>
/// <param name="input"></param>
/// <param name="key"></param>
/// <param name="reduce"></param>
/// <returns></returns>
public static Bag <KeyValuePair <K, T> > GroupBy <K, T>(
this Bag <T> input, Func <T, K> key, Func <T, T, T> reduce)
=> input.GroupBy(key, v => v, reduce);