fibonacci :: Int -> Int
-- testing 8 combinations of argument values
-- pruning with 9/13 rules
-- looking through 3 candidates of size 1
-- looking through 6 candidates of size 2
-- looking through 17 candidates of size 3
-- looking through 41 candidates of size 4
-- looking through 76 candidates of size 5
-- looking through 155 candidates of size 6
-- looking through 249 candidates of size 7
-- looking through 481 candidates of size 8
-- looking through 762 candidates of size 9
-- looking through 1413 candidates of size 10
-- looking through 2257 candidates of size 11
fibonacci 0  =  1
fibonacci 1  =  1
fibonacci x  =  fibonacci (dec x) + fibonacci (dec (dec x))