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:
fun => <coq-core.plugins.ltac::assumption@0>