Goal:
  
  G : t
  to_simple : forall (x y : V.t) (p : PathOf x y),
              is_simple p = true -> SPath (nodes p) x y
  x0 : V.t
  y0 : V.t
  p : PathOf x0 y0
  Hp : is_simple p = true
  x : V.t
  y : V.t
  z : V.t
  e : EdgeOf x y
  p0 : PathOf y z
  Hp0 : ~~ VSet.mem x (nodes p0) && is_simple p0 = true
  ============================
   (VSetProp.Add x (nodes p0) (VSet.add x (nodes p0)) /\
    ~ VSet.In x (nodes p0))


DEBUG:Going to execute:
match goal with
           | H:?X
             |- _ =>
                 match goal with
                 | H':?Y
                   |- _ =>
                       match H with
                       | H' => fail 2
                       | _ => unify X Y; clear H' || clear H
                       end
                 end
           end