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 \/ x = e..t)


DEBUG:Going to execute:
match goal with
           | |- ?X1 -> ?X2 => intro
           | |- ~ _ => unfold not at 1; intro
           end