Goal:
  
  G : t
  s0 : VSet.t
  s' : VSet.t
  x : V.t
  y : V.t
  z : V.t
  d : DisjointAdd x s0 s'
  e : EdgeOf x y
  p : SPath s0 y z
  u : VSet.elt
  Hu : {VSet.In u (snodes (spath_step d e p))} + {u = z}
  IHp : forall Hu : {VSet.In u (snodes p)} + {u = z},
        sweight (split p u Hu).1 + sweight (split p u Hu).2 = sweight p
  X : x <> u
  ============================
   (e.π1 + sweight p = e.π1 + sweight p)


DEBUG:Going to execute:
solve
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]