private static IReadOnlySet <V> Step(IReadOnlySet <V> activeCells, Func <V, IEnumerable <V> > getNear)
{
return(activeCells
.SelectMany(getNear)
.CountFrequency()
.Where(g => g.Value == 3 || g.Value == 2 && activeCells.Contains(g.Key))
.Select(g => g.Key)
.ToHashSet());
}
