Inductive Type_of_invariant_in_forward_for_should_be_environ_arrow_mpred_but_is : Type -> Prop := .Inductive Type_of_bound_in_forward_for_should_be_Z_but_is : Type -> Prop := .Ltac check_type_forward_for_simple_bound :=
   match goal with |- semax _ _ ?c _ => 
         let x := constr:(match c with (Ssequence _ (Sloop _ (Sset _ e))) => Some (typeof e) | _ => None end) in let x := eval hnf in x in let x := eval simpl in x in
         match x with
         | None => idtac
         | Some ?t => 
             unify (is_int32_type t) true
             + fail 100 "At present, forward_for_simple_bound works only on iteration variables that are (signed or unsigned) int, but your iteration variable has type" t
         end
     end.Ltac forward_for_simple_bound n Pre :=
  check_Delta; check_POSTCONDITION;
 repeat match goal with |-
      semax _ _ (Ssequence (Ssequence (Ssequence _ _) _) _) _ =>
      apply -> seq_assoc; abbreviate_semax
 end;
 match goal with |-
      semax _ _ (Ssequence (Ssequence (Sfor _ _ _ _) _) _) _ =>
      apply -> seq_assoc; abbreviate_semax
 | _ => idtac
 end;
 first [
    match type of n with
      ?t => first [ unify t Z | elimtype (Type_of_bound_in_forward_for_should_be_Z_but_is t)]
    end;
    match type of Pre with
      ?t => first [unify t (environ -> mpred); fail 1 | elimtype (Type_of_invariant_in_forward_for_should_be_environ_arrow_mpred_but_is t)]
    end
  | match goal with
    | |- semax _ _ (Sfor _ _ _ _) _ =>
           rewrite semax_seq_skip
    | |- semax _ _ (Ssequence _ (Sloop _ _)) _ =>
           rewrite semax_seq_skip
    | |- semax _ _ (Ssequence _ ?MORE_COMMANDS) _ =>
        revert MORE_COMMANDS;
        match goal with
        | |- let MORE_COMMANDS := @abbreviate _ (Sloop _ _) in _ =>
            intros MORE_COMMANDS;
            rewrite semax_seq_skip
        end
    | _ => idtac
    end;
    forward_for_simple_bound'' n Pre; [.. | abbreviate_semax; cbv beta; try fwd_skip]
  ].Fixpoint no_breaks (s: statement) : bool :=
 match s with
 | Sbreak => false
 | Ssequence a b => andb (no_breaks a) (no_breaks b)
 | Sifthenelse _ a b => andb (no_breaks a) (no_breaks b)
 | Sloop _ _ => true (* breaks within the inner loop are OK *)
 | _ => true
 end.

Lemma semax_skip_seq1: forall {Espec: OracleKind} {CS: compspecs} Delta P s1 s2 Q, semax Delta P (Ssequence s1 s2) Q -> semax Delta P (Ssequence (Ssequence Sskip s1) s2) Q.