public static ChannelReader<T> Skip<T>(this ChannelReader<T> reader,
                                            long count,
                                            ChannelParallelOptions options)

using Amphisbaena;
using Amphisbaena.Linq;

...

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

int sum = await data
  .ToChannelReader()
  .Skip(3) 
  .Aggregate((s, a) => s + a);