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:
<Coq.Init.Tauto.axioms> flags ||
             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 ||
               match goal with
               | |- _ -> _ => intro; t_tauto_intuit
               | |- _ => t_reduce; t_solver
               end || t_solver
Level 3: evaluation returns
<coq-core.plugins.tauto::is_conj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := x = e..s
of type
constr of type tacvalue

Level 3: Backtrace:

Tactic failure.
Level 3: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 2: Backtrace:

Tactic failure.
Level 1: Backtrace:

No matching clauses for match.
Level 1: Backtrace:

No matching clauses for match.