piSet :: (Eq a) => [a] -> [[[a]]]
piSet r = [ a | a <- (powerset' . powerset') r,
          (null [ a | x <- a, y <- a,
                  x /= y &&
                  (length x <= length y) && (subset x y)])]
piSet :: (Eq a) => [a] -> [[[a]]]
piSet r = [ a | a <- (powerset' . powerset') r,
          (null [ a | x <- a, y <- a,
                  x /= y &&
                  (length x <= length y) && (subset x y)])]
piSet r = [ a | a <- (powerset' . powerset') r,
          (null [ a | x <- a, y <- a,
                  x /= y &&
                  (length x <= length y) && (subset x y)])]