duplicates :: [Int] -> [Int]
-- testing 6 combinations of argument values
-- pruning with 21/26 rules
-- looking through 2 candidates of size 1
-- looking through 1 candidates of size 2
-- looking through 1 candidates of size 3
-- looking through 2 candidates of size 4
-- looking through 1 candidates of size 5
-- looking through 2 candidates of size 6
-- looking through 3 candidates of size 7
-- looking through 8 candidates of size 8
-- looking through 16 candidates of size 9
-- looking through 25 candidates of size 10
-- looking through 36 candidates of size 11
-- looking through 65 candidates of size 12
-- looking through 141 candidates of size 13
-- looking through 322 candidates of size 14
-- looking through 644 candidates of size 15
-- looking through 1189 candidates of size 16
-- looking through 2189 candidates of size 17
-- tested 2599 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
-- looking through 2 candidates of size 1
-- looking through 1 candidates of size 2
-- looking through 1 candidates of size 3
-- looking through 2 candidates of size 4
-- looking through 1 candidates of size 5
-- looking through 2 candidates of size 6
-- looking through 3 candidates of size 7
-- looking through 8 candidates of size 8
-- looking through 16 candidates of size 9
-- looking through 25 candidates of size 10
-- looking through 36 candidates of size 11
-- looking through 65 candidates of size 12
-- looking through 141 candidates of size 13
-- looking through 322 candidates of size 14
-- looking through 644 candidates of size 15
-- looking through 1189 candidates of size 16
-- looking through 2189 candidates of size 17
-- tested 2599 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
-- looking through 1 candidates of size 1
-- looking through 0 candidates of size 2
-- looking through 2 candidates of size 3
-- looking through 5 candidates of size 4
-- looking through 15 candidates of size 5
-- looking through 25 candidates of size 6
-- looking through 59 candidates of size 7
-- looking through 108 candidates of size 8
-- looking through 256 candidates of size 9
-- looking through 458 candidates of size 10
-- looking through 1064 candidates of size 11
-- looking through 1948 candidates of size 12
-- looking through 4516 candidates of size 13
-- looking through 8304 candidates of size 14
-- tested 8844 candidates
positionsFrom x y []  =  []
positionsFrom x y (z:xs)  =  (if y == z then (x :) else id) (positionsFrom (x + 1) y xs)