>>>>>
>>>>>Off topic but since I think you enjoy obscure code I just wrote this (purely as a mockup):

static IEnumerable<int> GetSequence()
>>>>>        {
>>>>>            bool b = false;
>>>>>            int x1 = 0;
>>>>>            int x2 = 1;
>>>>>
>>>>>            while (true)
>>>>>            {
>>>>>                b=!b;
>>>>>                yield return b ? x1 += x2 : x2 += x1;
>>>>>            }
>>>>>        ]

>>>>
>>>>working my way through it with a pencil
>>>>
>>>>1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
>>>>
>>>>
>>>>If we number them as n0, n1, n2, n3, n4, ....
>>>>
>>>>then n[i] = n[i-1] + n[i-2]
>>>
>>>yep. Next in the series is the sum of the previous two items. It's called the Fibonacci series - but I actually got it wrong. It should be:
>>>0,1,1,2,3,5,8 etc
>>
>>
>>Take -1 as starting value for x1
>>
>>

>>  int x1 = -1;
>>  int x2 = 1;
>>

>>
>>As an alternative, you can do the calculation after the yield return
>
>Or maybe just:

int x1 = -1;
>int x2 = 1;
>while (true)
>      {
>           yield return x1 += x2;
>           yield return x2 += x1;
>       }

Excellent - no test any more