Goal:
    G : t  H : invariants G  x : V.t  y : V.t  e : EdgeOf G x y
  ============================
   (VSet.In (x, e.π1, y)..s (V G) ->
    VSet.In (x, e.π1, y)..t (V G) -> VSet.In x (V G))


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 ])