Program Fixpoint addToTree (st : SeedTree) (p : SplitPath) (f : SplitPath -> RandomSeed)
        (l : list SplitPath)
        (Corr : correspondingSeedTree l f st)
        (Pref : forall p', In p' l -> (forall p1 p2, p ++ p1 = p' ++ p2 -> False))
: SeedTree :=
  match st with
    | SeedTreeUndef =>
      match p with
        | [] => SeedTreeLeaf (f p)
        | Left  :: p' => SeedTreeNode (addToTree SeedTreeUndef p' (refineFunction f Left) [] _ _) SeedTreeUndef
        | Right :: p' => SeedTreeNode SeedTreeUndef (addToTree SeedTreeUndef p' (refineFunction f Right) [] _ _)
      end
    | SeedTreeLeaf s => _  (* Contradiction! *)
    | SeedTreeNode st1 st2 =>
      match p with
        | [] => SeedTreeLeaf (f p)
        | Left  :: p' => SeedTreeNode (addToTree st1 p' (refineFunction f Left) (refineList Left l) _ _) st2
        | Right :: p' => SeedTreeNode st1 (addToTree st2 p' (refineFunction f Right) (refineList Right l) _ _)
      end
  end.

Lemma PrefixFreeTail : forall a l, PrefixFree (a :: l) -> PrefixFree l.