permutation, perm, powerSet

Clean code for sets

permutation :: [a] -> [[a]] | Eq a
permutation [] = [[]]
permutation xs = [[x:rest] \\ x <- xs, rest <- (permutation o filter ((<>) x)) xs]

perm :: [[a]] -> [[a]] | Eq a
perm [] = [[]]
perm [xs:xss] = [[x:rest] \\ x <- xs, rest <- (perm o map (filter ((<>) x))) xss]

powerSet :: [a] -> [[a]]
powerSet [] = [[]]
powerSet [x:xs] = flatten [[[x:ys],ys] \\ ys <- powerSet xs]