public static async Task<T> Aggregate<T, A, S>(this ChannelReader<S> reader,
                                                    A seed,
                                                    Func<A, S, A> accumulate,
                                                    Func<A, T> result,
                                                    ChannelParallelOptions options)

using Amphisbaena;
using Amphisbaena.Linq;

...

int[] data = new int[] {1, 2, 3};

long sum = await data
  .ToChannelReader()
  .Aggregate(0L, (s, a) => s + a);