Goal:
    G : t  HI : invariants G  HG : acyclic_no_loop G  z : Z
  ============================
   (sweight (spath_refl G (V G) (s G)) = z ->
    ~ VSet.In (s G) (V G) -> let H := CoreTactics.block in z = 0)


DEBUG:Going to execute:
match goal with
           | |- let _ := CoreTactics.block in _ => intros _
           | |- _ => try (intro; CoreTactics.intros_until_block)
           end