Parameter REACH : mem -> (block -> bool) -> block -> bool.Axiom REACHAX : (* Constructible via FiniteMaps.v, relying on finiteness of memories *)
  forall m B b, REACH m B b = true
  <-> exists L, reach m (fun bb => B bb = true) L b.

Lemma REACH_nil: forall m B b, B b = true -> REACH m B b = true.