Section extensions.

Lemma safe_step_forward {Espec: OracleKind}:
forall psi n ora st m,
cl_at_external st = None ->
jsafeN (@OK_spec Espec) psi (S n) ora st m ->
exists st', exists m',
jstep (cl_core_sem psi) st m st' m' /\ jm_bupd ora (jsafeN (@OK_spec Espec) psi n ora  st') m'.