Infinite loop in an imperative language

It is possible to define the abstract syntax and the semantics of a little programming language with the following inductive definitions:

Section little_semantics.
 Variables Var aExp bExp : Set.
 Inductive inst : Set :=
 | Skip : inst
 | Assign : Var->aExp->inst
 | Sequence : inst->inst->inst
 | WhileDo : bExp->inst->inst.

  (state : Set)
  (update : state->Var->Z -> option state)
  (evalA : state->aExp -> option Z)
  (evalB : state->bExp -> option bool).

 Inductive exec : state->inst->state->Prop :=
 | execSkip : forall s:state, exec s Skip s
 | execAssign :
    forall (s s1:state)(v:Var)(n:Z)(a:aExp),
     evalA s a = Some n -> update s v n = Some s1 ->
     exec s (Assign v a) s1
 | execSequence :
    forall (s s1 s2:state)(i1 i2:inst),
     exec s i1 s1 -> exec s1 i2 s2 ->
     exec s (Sequence i1 i2) s2
 | execWhileFalse :
    forall (s:state)(i:inst)(e:bExp),
     evalB s e = Some false -> exec s (WhileDo e i) s
 | execWhileTrue :
    forall (s s1 s2:state)(i:inst)(e:bExp),
     evalB s e = Some true ->
     exec s i s1 ->
     exec s1 (WhileDo e i) s2 ->
     exec s (WhileDo e i) s2.

Prove that executing a while loop with a body that does not change its state and a conditional test that is true never succeeds to produce a final state. Here is the exact statement:

 forall (s s':state)(b:bExp),
   exec s (WhileDo b Skip) s' -> evalB s b = Some true -> False.


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Pierre Castéran