Goal:
  
  G1 : t
  ext : EdgeSet.t
  x : VSet.elt
  e : Edge.t
  s : VSet.t
  _a_ : x = e..t
  _b_ : VSet.In x (V G1)
  ============================
   (x = e..s)


DEBUG:Going to execute:
constructor 1
Level 5: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := x = e..s
of type
constr of type tacvalue