Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  e : Edge.t
  s : VSet.t
  _a_ : x = e..s
  _b_ : VSet.In x (V G1)
  ============================
   (x = e..s /\ VSet.In x (V G1) \/
    x = e..t /\ VSet.In x (V G1) \/ VSet.In x 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