public static ChannelReader<T> Where<T>(this ChannelReader<T> reader,
                                             Func<T, long, bool> condition,
                                             ChannelParallelOptions options)

using Amphisbaena;
using Amphisbaena.Linq;

...

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

double fraction = await data
  .ToChannelReader()
  .Where((value, index) => value > 2 && index % 2 == 0)
  .CountFraction(item => item < 6);