duplicates :: [Int] -> [Int]
-- testing 6 combinations of argument values
-- pruning with 21/26 rules
-- 2 candidates of size 1
-- 1 candidates of size 2
-- 0 candidates of size 3
-- 2 candidates of size 4
-- 1 candidates of size 5
-- 2 candidates of size 6
-- 3 candidates of size 7
-- 8 candidates of size 8
-- 16 candidates of size 9
-- 25 candidates of size 10
-- 36 candidates of size 11
-- 63 candidates of size 12
-- 133 candidates of size 13
-- 299 candidates of size 14
-- 602 candidates of size 15
-- 1116 candidates of size 16
-- 2031 candidates of size 17
-- tested 2450 candidates
duplicates []  =  []
duplicates (x:xs)  =  if elem x xs && not (elem x (duplicates xs))
                      then x:duplicates xs
                      else duplicates xs

duplicates :: [Int] -> [Int]
-- pruning with 21/26 rules
-- 2 candidates of size 1
-- 1 candidates of size 2
-- 0 candidates of size 3
-- 2 candidates of size 4
-- 1 candidates of size 5
-- 2 candidates of size 6
-- 3 candidates of size 7
-- 8 candidates of size 8
-- 16 candidates of size 9
-- 25 candidates of size 10
-- 36 candidates of size 11
-- 63 candidates of size 12
-- 133 candidates of size 13
-- 299 candidates of size 14
-- 602 candidates of size 15
-- 1116 candidates of size 16
-- 2031 candidates of size 17
-- tested 2450 candidates
duplicates []  =  []
duplicates (x:xs)  =  if elem x xs && not (elem x (duplicates xs))
                      then x:duplicates xs
                      else duplicates xs

positionsFrom :: Int -> A -> [A] -> [Int]
-- testing 360 combinations of argument values
-- pruning with 5/10 rules
-- 1 candidates of size 1
-- 0 candidates of size 2
-- 2 candidates of size 3
-- 5 candidates of size 4
-- 9 candidates of size 5
-- 16 candidates of size 6
-- 43 candidates of size 7
-- 73 candidates of size 8
-- 185 candidates of size 9
-- 285 candidates of size 10
-- 722 candidates of size 11
-- 1157 candidates of size 12
-- 2847 candidates of size 13
-- 4505 candidates of size 14
-- tested 5484 candidates
positionsFrom x y []  =  []
positionsFrom x y (z:xs)  =  (if y == z then (x :) else id) (positionsFrom (x + 1) y xs)