Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  v : VSet.elt
  s : VSet.t
  E : VSet.mem v (V G1) = false
  H0 : x = v /\ VSet.In x (V G1)
  ============================
   (VSet.In x s)


DEBUG:Going to execute:
elim id
Level 2: evaluation returns
<coq-core.plugins.tauto::is_conj@0>
where
tauto_flags := 
of type
tauto_flags
X1 := x = v /\ VSet.In x (V G1)
of type
constr of type tacvalue