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
  H : VSet.In y0 (V G1) -> first_in q = y0
  Heq : VSet.mem x0 (V G1) = false
  H0 : 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.