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 : lsp G x_0 y_0 = Some K
  x : V.t
  p' : SPath G' (V G') y_0 x
  p1 : SPath G (V G') y_0 y_0
  p2 : SPath G (V G') x_0 x
  He : sweight p' <= sweight p1 - K + sweight p2
  x0x : Z
  lspx0x : lsp G x_0 x = Some x0x
  wp2 : sweight p2 <= x0x
  lspy0 : Some 0 = Some 0
  wp1 : sweight p1 <= 0
  ============================
   (sweight p' <= - K + x0x)


DEBUG:Going to execute:
Zify.zify_pre_hook