Definition recompose (tt : (canonTree*canonTree)): canonTree :=
   let (t1, t2) := tt in
   let (x1, c1) := t1 in
   let (x2, c2) := t2 in
   match x1 as s return (canonicalTree s -> canonTree) with
   | Leaf b1 =>
       fun c3 : canonicalTree (Leaf b1) =>
       match x2 as s return (canonicalTree s -> canonTree) with
       | Leaf b2 =>
           fun c4 : canonicalTree (Leaf b2) =>
           match bool_dec b1 b2 with
           | left e =>
               let H :=
                 eq_rec b1
                   (fun b3 : bool => canonicalTree (Leaf b3) -> canonTree)
                   (fun _ : canonicalTree (Leaf b1) =>
                    exist (fun t : ShareTree => canonicalTree t) (Leaf b1) c3)
                   b2 e in
               H c4
           | right n =>
               exist (fun t : ShareTree => canonicalTree t)
                 (Node (Leaf b1) (Leaf b2)) (compose_canon1 b1 b2 n c3 c4)
           end
       | Node x21 x22 =>
           fun c4 : canonicalTree (Node x21 x22) =>
           exist (fun t : ShareTree => canonicalTree t)
                (Node (Leaf b1) (Node x21 x22)) (compose_canon2 b1 x21 x22 c3 c4)
       end c2
   | Node x11 x12 =>
       fun c3 : canonicalTree (Node x11 x12) =>
       match x2 as s return (canonicalTree s -> canonTree) with
       | Leaf b2 =>
           fun c4 : canonicalTree (Leaf b2) =>
           exist (fun t : ShareTree => canonicalTree t)
             (Node (Node x11 x12) (Leaf b2)) (compose_canon3 x11 x12 b2 c3 c4)
       | Node x21 x22 =>
           fun c4 : canonicalTree (Node x21 x22) =>
           exist (fun t : ShareTree => canonicalTree t)
             (Node (Node x11 x12) (Node x21 x22))
             (compose_canon4 x11 x12 x21 x22 c3 c4)
       end c2
   end c1.

Lemma tree_round_left_join : forall n t1 t2 t3 t1' t2' t3', join t1 t2 t3 -> roundL n t1 = Some t1' -> roundL n t2 = Some t2' -> roundL n t3 = Some t3' -> join t1' t2' t3'.