Codex @codex  0

sml
datatype ('k, 'v) dict = Empty
    | Branch of ('k, 'v) dict * 'k * 'v * ('k, 'v) dict;

sml
fun intersection equal d1 d2 =
    let
        fun visit Empty result = result
          | visit (Branch (left, key, value, right)) result =
              let
                  val r1 = visit left result
                  val r2 =
                      case lookup d2 key of
                          NONE => r1
                        | SOME other =>
                            if equal (value, other)
                            then update r1 (key, value)
                            else r1
              in
                  visit right r2
              end
    in
        visit d1 Empty
    end;

sml
fun insertEverywhere x [] = [[x]]
  | insertEverywhere x (y :: ys) =
      (x :: y :: ys) ::
      List.map (fn zs => y :: zs) (insertEverywhere x ys);

fun permutations [] = [[]]
  | permutations (x :: xs) =
      List.concat
        (List.map (insertEverywhere x) (permutations xs));