Goal:
  
  G1 : t
  G2 : t
  H : invariants G1
  ext := EdgeSet.diff (E G2) (E G1) : EdgeSet.t
  bs := border_set G1 ext : VSet.t
  s : VSet.t
  y : V.t
  z : V.t
  p : SPath G2 s y z
  _s2_ : VSet.t
  _x0_ : V.t
  _z0_ : V.t
  _s1_ : VSet.t
  _y0_ : V.t
  _d_ : DisjointAdd _x0_ _s1_ _s2_
  _e_ : EdgeOf G2 _x0_ _y0_
  _q_ : SPath G2 _s1_ _y0_ _z0_
  __view_subject_1_ : VSet.In _x0_ (V G1)
  _x_ : V.t
  _projT1_ : Z
  _hz_ : VSet.In _z0_ (V G1)
  e := (_x_, _projT1_, _x0_) : (V.t × Z) × V.t
  _he_ : EdgeSet.In e ext
  ============================
   (_x0_ = e..s \/ _x0_ = e..t)


DEBUG:Going to execute:
Ltac.right; t_tauto_intuit
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Proof is not complete.