public static IEnumerable <T> Choice <T>(T[] source, int k)
{
IndexList pastIndices = new IndexList();
for (int i = 0; i < k; i++)
{
int nextIndex = Rng.Next(source.Length - i);
for (int j = 0; j < i; j++)
{
if (nextIndex >= pastIndices[j])
{
nextIndex++;
}
}
pastIndices.InsertSorted(nextIndex);
yield return(source[nextIndex]);
}
}