fibonacci :: Int -> Int
-- testing 7 combinations of argument values
-- pruning with 21/44 rules
-- looking through 4 candidates of size 1
-- looking through 3 candidates of size 2
-- looking through 9 candidates of size 3
-- looking through 9 candidates of size 4
-- looking through 30 candidates of size 5
-- looking through 30 candidates of size 6
-- looking through 156 candidates of size 7
-- looking through 157 candidates of size 8
-- looking through 813 candidates of size 9
-- looking through 797 candidates of size 10
-- looking through 4449 candidates of size 11
-- looking through 4438 candidates of size 12
-- tested 6539 candidates
fibonacci 0  =  1
fibonacci 1  =  1
fibonacci x  =  fibonacci (x - 1) + fibonacci (x - 2)