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:
<coq-core.plugins.ssreflect::ssrmove@1> $1 $2