Goal:
  
  G : t
  s : VSet.t
  y : VSet.elt
  s0 : VSet.t
  s1 : VSet.t
  x : V.t
  y' : V.t
  z : V.t
  disj01 : DisjointAdd x s0 s1
  e : EdgeOf G x y'
  p : SPath G s0 y' z
  ih : VSet.In y (snodes G p) ->
       forall (s' : VSet.t) (q : SPath G s' z y'),
       Disjoint s0 s' ->
       exists c : SPath G (VSet.union s0 s') y y,
         sweight c = sweight p + sweight q
  s' : VSet.t
  q : SPath G s' z x
  disj' : Disjoint s1 s'
  ============================
   (e.π1 + sweight p + sweight q = e.π1 + sweight p + sweight q)


DEBUG:Going to execute:
trivial; hnf; intros; (solve
            [ do
            ![ solve
             [ trivial | apply : eq_sym ; trivial ]
             | discriminate
             | contradiction
             | Ltac.split ]
            | case not_locked_false_eq_true; Ltac.assumption
            | match goal with
              | H:~ _ |- _ => solve [ case H; trivial ]
              end ])