Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  y_0 : V.t
  x_0 : V.t
  Hxs : lsp0 G (V G) x_0 y_0 = None
  K : Z
  z : V.t
  q1 : PathOf G x_0 y_0
  q : PathOf G' x_0 z
  e : EdgeOf G' y_0 x_0
  q2 : PathOf G x_0 z
  H0 : K = e.π1
  XX0 : (y_0, K, x_0) = (y_0, e.π1, x_0)
  IHq : weight q = weight q1 + K + weight q2
  XX := lsp0_spec_le G (simplify2' G q1)
     : (Some (sweight (simplify2' G q1)) <= lsp0 G (V G) x_0 y_0)%nbar
  ============================
   (e.π1 + (weight q1 + K + weight q2) = K + weight q2)


DEBUG:Going to execute:
rewrite -> Hxs in XX