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


DEBUG:Going to execute:
fun => <coq-core.plugins.ltac::split@0>