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


DEBUG:Going to execute:
trivial; hnf; intros