Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  y_0 : V.t
  x_0 : V.t
  Vx : VSet.In x_0 (V G)
  Vy : VSet.In y_0 (V G)
  K : Z
  Hxs : lsp0 G (V G) x_0 y_0 = Some K
  x : VSet.elt
  Hx : VSet.In x (V G')
  p : SPath G' (V G) x x
  H : 0 <= sweight p
  q1 : SPath G (V G) x y_0
  q2 : SPath G (V G) x_0 x
  XX : sweight p <= sweight q1 - K + sweight q2
  Hle : sweight (simplify2' G (concat G (to_pathOf G q2) (to_pathOf G q1))) <=
        K
  H0 : sweight q2 + sweight q1 <=
       sweight (simplify2' G (concat G (to_pathOf G q2) (to_pathOf G q1)))
  H1 : sweight q1 - K + sweight q2 <= 0
  ============================
   (sweight p = 0)


DEBUG:Going to execute:
zify_op