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 11 candidates of size 4
-- looking through 34 candidates of size 5
-- looking through 41 candidates of size 6
-- looking through 187 candidates of size 7
-- looking through 204 candidates of size 8
-- looking through 957 candidates of size 9
-- looking through 1059 candidates of size 10
-- looking through 5295 candidates of size 11
-- looking through 6052 candidates of size 12
-- tested 8126 candidates
fibonacci 0  =  1
fibonacci 1  =  1
fibonacci x  =  fibonacci (x - 1) + fibonacci (x - 2)