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 \/ x = e..t)


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