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 26 candidates of size 5
-- looking through 27 candidates of size 6
-- looking through 116 candidates of size 7
-- looking through 120 candidates of size 8
-- looking through 552 candidates of size 9
-- looking through 540 candidates of size 10
-- looking through 2663 candidates of size 11
-- looking through 2675 candidates of size 12
-- tested 4147 candidates
fibonacci 0  =  1
fibonacci 1  =  1
fibonacci x  =  fibonacci (x - 1) + fibonacci (x - 2)