isLeft :: Either A A -> Bool
-- testing 4 combinations of argument values
-- pruning with 0/0 rules
-- 2 candidates of size 1
-- 1 candidates of size 2
-- tested 3 candidates
isLeft (Left x)  =  True
isLeft (Right x)  =  False

isRight :: Either A A -> Bool
-- testing 4 combinations of argument values
-- pruning with 0/0 rules
-- 2 candidates of size 1
-- 1 candidates of size 2
-- tested 3 candidates
isRight (Left x)  =  False
isRight (Right x)  =  True

fromLeft :: A -> Either A A -> A
-- testing 5 combinations of argument values
-- pruning with 0/0 rules
-- 1 candidates of size 1
-- 1 candidates of size 2
-- tested 2 candidates
fromLeft x (Left y)  =  y
fromLeft x (Right y)  =  x

fromLeft :: A -> Either A A -> A
-- testing 5 combinations of argument values
-- pruning with 0/0 rules
-- 1 candidates of size 1
-- 1 candidates of size 2
-- tested 2 candidates
fromLeft x (Left y)  =  x
fromLeft x (Right y)  =  y

either :: (A -> A) -> (A -> A) -> Either A A -> A
-- pruning with 0/0 rules
-- 1 candidates of size 1
-- 1 candidates of size 2
-- 4 candidates of size 3
-- 12 candidates of size 4
-- tested 12 candidates
either f g (Left x)  =  f x
either f g (Right x)  =  g x

lefts :: [Either A A] -> [A]
-- testing 5 combinations of argument values
-- pruning with 0/0 rules
-- 1 candidates of size 1
-- 0 candidates of size 2
-- tested 1 candidates
lefts  =  undefined  -- search exhausted
-- could not find implementation using only
-- Left, Right, False, True, [], and (:)
-- consider increasing target/maxSize or refining the ingredients

