Module NonStan. Section Nonstan.
Variable R : BSRS.
Context {ff : falsity_flag}.
Instance IB : interp (option (string bool)).

Lemma IB_deriv' rho t1 t2 u v :
eval rho t1 = Some u -> eval rho t2 = Some v ->
rho ⊨ (Pr t1 t2) <-> derivable R u v.