Section Reduction.
Variables (M : SMN) (L R L' R': list Symbol) (X Y: State).
Variable confluent_M : confluent M.
Arguments confluent_M {X Y1 Y2}.
Variable length_preserving_M : length_preserving M.
Variable XY_neq : X <> Y.
Variable Y_fresh : Forall (fun '((_, _, x), (_, _, y)) => x <> Y /\ y <> Y) M.
Variable lp_XY : length (L++R) = length (L'++R') /\ 1 <= length (L++R).
Definition M' : SMN :=
[((L, R, X), (L', R', Y)); ((L', R', Y), (L, R, X))] ++ M.
Definition get_state (c: Config) := match c with (_, _, x) => x end.
Definition valid_state (x: State) := x <> Y.

Lemma synchronize_step {x y} : step M' x y ->
exists z, reachable_n M' 1 x z /\ reachable_n M' 1 z y /\ valid_state (get_state z).