fibonacci :: Int -> Int
-- testing 7 combinations of argument values
-- pruning with 21/44 rules
-- looking through 4 candidates of size 1
-- looking through 9 candidates of size 2
-- looking through 20 candidates of size 3
-- looking through 87 candidates of size 4
-- looking through 127 candidates of size 5
-- looking through 420 candidates of size 6
-- looking through 845 candidates of size 7
-- looking through 2136 candidates of size 8
-- looking through 5185 candidates of size 9
-- looking through 13000 candidates of size 10
-- looking through 33460 candidates of size 11
-- looking through 85901 candidates of size 12
fibonacci 0 = 1
fibonacci 1 = 1
fibonacci x = fibonacci (x - 1) + fibonacci (x - 2)