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


DEBUG:Going to execute:
change (let _ := CoreTactics.block in T)