public static ChannelReader<T> SelectMany<T, S>(this ChannelReader<S> reader,
                                                     Func<S, long, ChannelReader<T>> selector,
                                                     ChannelParallelOptions options)

using Amphisbaena;
using Amphisbaena.Linq;

...
int[][] data = new int[][] {
  new int[] { 1, 2, 3 },
  new int[] { 4 },
  new int[] { 5, 6, 7, 8 },
  new int[] { 9, 10},
};

int total = await data
  .ToChannelReader()
  .SelectMany(line => line.ToChannelReader())
  .Aggregate((s, a) => s + a);