Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  e : EdgeSet.elt
  es : list EdgeSet.elt
  H0 : VSet.In x (V G1)
  H1 : VSet.In x (fold_right fold_fun VSet.empty es) ->
       exists e : EdgeSet.elt,
         In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)
  H2 : (exists e : EdgeSet.elt,
          In e es /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)) ->
       VSet.In x (fold_right fold_fun VSet.empty es)
  H3 : x = e..t
  ============================
   (x = e..s)


DEBUG:Going to execute:
match reverse goal with
           | id:(?X1 -> ?X2) -> ?X3
             |- _ =>
                 Ltac.cut X3;
                  [ intro; clear id; t_tauto_intuit
                  | Ltac.cut (X1 -> X2);
                     [ exact
                     id
                     | generalize (fun y : X2 => id (fun x : X1 => y)); intro;
                        clear id; (solve [ t_tauto_intuit ]) ] ]
           | id:~ ?X1 -> ?X3
             |- _ =>
                 Ltac.cut X3;
                  [ intro; clear id; t_tauto_intuit
                  | Ltac.cut (~ X1);
                     [ exact
                     id
                     | clear id; intro; (solve [ t_tauto_intuit ]) ] ]
           | |- ?X1 =>
                 <Coq.Init.Tauto.is_disj> flags X1; (solve
                  [ Ltac.left; t_tauto_intuit | Ltac.right; t_tauto_intuit ])
           end