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
  s0 : VSet.t
  s' : VSet.t
  z : V.t
  d : DisjointAdd y_0 s0 s'
  q1 : SPath G s0 x_0 y_0
  q : SPath G' s0 x_0 z
  e : EdgeOf G' y_0 x_0
  q2 : SPath G s0 x_0 z
  H0 : K = e.π1
  XX0 : (y_0, K, x_0) = (y_0, e.π1, x_0)
  IHq : sweight q = sweight q1 + K + sweight q2
  XX := lsp0_spec_le G (simplify2' G (to_pathOf G q1))
     : (Some (sweight (simplify2' G (to_pathOf G q1))) <=
        lsp0 G (V G) x_0 y_0)%nbar
  ============================
   (e.π1 + sweight q = K + sweight q2)


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