fibonacci :: Int -> Int
-- testing 7 combinations of argument values
-- pruning with 21/44 rules
-- 4 candidates of size 1
-- 3 candidates of size 2
-- 9 candidates of size 3
-- 9 candidates of size 4
-- 26 candidates of size 5
-- 27 candidates of size 6
-- 116 candidates of size 7
-- 120 candidates of size 8
-- 552 candidates of size 9
-- 538 candidates of size 10
-- 2661 candidates of size 11
-- 2661 candidates of size 12
-- tested 4133 candidates
fibonacci 0  =  1
fibonacci 1  =  1
fibonacci x  =  fibonacci (x - 1) + fibonacci (x - 2)