Goal:
    G1 : t  ext : EdgeSet.t  x : VSet.elt
  ============================
   ((exists e : EdgeSet.elt,
       In e (EdgeSet.elements ext) /\
       (x = e..s \/ x = e..t) /\ VSet.In x (V G1)) <->
    (exists e : EdgeSet.elt,
       EdgeSet.In e ext /\ (x = e..s \/ x = e..t) /\ VSet.In x (V G1)))


DEBUG:Going to execute:
split; move  =>[e [? ?]]; exists e