Remark int_ltu_2_inv:
  forall y z,
  Int.ltu y iwordsize' = true ->
  Int.ltu z iwordsize' = true ->
  Int.unsigned (Int.add y z) <= Int.unsigned iwordsize' ->
  let y' := repr (Int.unsigned y) in
  let z' := repr (Int.unsigned z) in
     Int.unsigned y = unsigned y'
  /\ Int.unsigned z = unsigned z'
  /\ ltu y' iwordsize = true
  /\ ltu z' iwordsize = true
  /\ Int.unsigned (Int.add y z) = unsigned (add y' z')
  /\ add y' z' = repr (Int.unsigned (Int.add y z)).

Theorem shl'_shl': forall x y z, Int.ltu y iwordsize' = true -> Int.ltu z iwordsize' = true -> Int.ltu (Int.add y z) iwordsize' = true -> shl' (shl' x y) z = shl' x (Int.add y z).