Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  y_0 : V.t
  x_0 : V.t
  K : Z
  Hxs : lsp G x_0 y_0 = Some K
  z : V.t
  e : EdgeOf G' y_0 x_0
  q : PathOf G' x_0 z
  q' : PathOf G x_0 z
  IHq : weight q = weight q'
  XX : (y_0, - K, x_0) = (y_0, e.π1, x_0)
  H0 : - K = e.π1
  H1 : x_0 = x_0
  ============================
   (e.π1 + weight q <= - K + weight q')


DEBUG:Going to execute:
destruct e as [e He]