fact: Example ex_bwd i x1 x2 (e : unit -> spec nat) q: verify (e tt) (i :+ (x1 :-> 1 :+ x2 :-> 4)) q -> verify (bind_s (write_s x2 4) e) (i :+ (x1 :-> 1 :+ x2 :-> 2)) q.
imports: mathcomp(ssreflect ssrbool ssrfun) LemmaOverloading(heaps stmod stsep stlog)