fib(6) = 8, total calls = 25
calls to F(k), k = 0..6: F(0)=5 F(1)=8 F(2)=5 F(3)=3 F(4)=2 F(5)=1 F(6)=1

n    F(n)          calls         ms          calls/prev
20   6765          21891         0.05        
22   17711         57313         0.12        2.618     
24   46368         150049        0.34        2.618     
26   121393        392835        0.86        2.618     
28   317811        1028457       2.21        2.618     
30   832040        2692537       5.83        2.618     
32   2178309       7049155       16.27       2.618     
34   5702887       18454929      40.88       2.618     
36   14930352      48315633      108.97      2.618     
38   39088169      126491971     280.85      2.618     
40   102334155     331160281     735.49      2.618     
42   267914296     866988873     1943.29     2.618
