Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  x : V.t
  z : Z
  p : SPath G (V G) x (s G)
  wp : sweight p = z
  H0 : ~ VSet.In x (V G)
  ============================
   (let H := CoreTactics.block in z = 0)


DEBUG:Going to execute:
fun id' =>
             match id' with
             | id => idtac
             | _ => revert id'; CoreTactics.revert_until id
             end