Goal:
  
  G1 : t
  G2 : t
  s : VSet.t
  x : V.t
  y : V.t
  p : SPath G2 s x y
  _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_
  _Hyp_ : VSet.In _y0_ (V G1) -> first_in _q_ = _y0_
  __view_subject__ : VSet.mem _x0_ (V G1) = false
  _view_subject_ : ~ VSet.In _x0_ (V G1)
  H : VSet.In _x0_ (V G1)
  ============================
   (first_in _q_ = _x0_)


DEBUG:Going to execute:
<coq-core.plugins.ltac::discriminate@0>
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.
Level 0: Backtrace:

Proof is not complete.