Lazy Fibonacci and Friends

gistfile1.cs

public static class SequenceEx
{
    public static IEnumerable<T> StartWith<T>(params T[] seeds)
    {
        return new List<T>(seeds).AsEnumerable();
    }
    
    public static IEnumerable<T> ThenFollow<T>(this IEnumerable<T> thisSequence, 
        Func<T,T,T> rule) where T:IEquatable<T>
    {
        while(true)
        {
            T last = thisSequence.ElementAt(thisSequence.Count () - 1);
            T lastButOne = thisSequence.ElementAt(thisSequence.Count () - 2);
        
            thisSequence = thisSequence
                                   .Concat((new List<T>()
                                   {rule.Invoke(last,lastButOne)}).AsEnumerable());
            yield return  rule.Invoke(last,lastButOne); 
                          
        }
    }
    public static IEnumerable<T> ThenFollow<T>(this IEnumerable<T> thisSequence, Func<T,T> rule)
        where T:IEquatable<T>
     {
        while(true)
        {
            T last = thisSequence.ElementAt(thisSequence.Count () - 1);
            
            thisSequence = thisSequence.Concat((new List<T>()
                                            {rule.Invoke(last)}).AsEnumerable());
            yield return  rule.Invoke(last); 
                          
        }
    }    
}
//Now add the following code in the Main() method:
void Main()
{
    Func<long,long,long> A015531Rule = (x,y) =>  4 *x + 5*y; 
    Func<long,long,long> fibonacciRule = (x,y) => x + y;
    Func<double,double> arbitraryRule = (x) => 1/(x + 1/x);
    SequenceEx.StartWith<long>(0,1) 
                        .ThenFollow(A015531Rule)
                        .Take(5)
                        .Dump("A015531");

    SequenceEx.StartWith<long>(1,1)
                         .ThenFollow(fibonacciRule)
                         .Take(40)
                         .Dump("First few Fibonacci Numbers");

    SequenceEx.StartWith(1.0)
                        .ThenFollow(arbitraryRule)
                       .Take(5)
                       .Dump("Arbitrary Sequence");
}