Goal:
  
  G : t
  x : V.t
  y : V.t
  z : V.t
  e : EdgeOf x y
  p : PathOf y z
  IHp : SPath (nodes p) y z
  Hx : VSet.mem x (snodes IHp) = false
  ============================
   (VSet.mem x (snodes IHp) = false)


DEBUG:Going to execute:
solve
[ trivial
with eq_true
| reflexivity
| symmetry; trivial
| contradiction
| congruence ]