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
  x : V.t
  y : V.t
  z : V.t
  eo : EdgeOf G2 x y
  p : SPath G2 s y z
  e := (x, eo.π1, y) : (V.t × Z) × V.t
  hx : VSet.In x (V G1)
  hz : VSet.In z (V G1)
  heo : ~ EdgeSet.In e (E G1)
  he : EdgeSet.In e ext
  ============================
   ((x = e..s \/ x = e..t) /\ VSet.In x (V G1))


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

Cannot apply lemma eq_sym
Level 0: Backtrace:

Cannot apply lemma eq_sym
Level 0: Backtrace:

Cannot apply lemma eq_sym