Goal:
  
  G : t
  HI : invariants G
  HG : acyclic_no_loop G
  z : V.t
  x : V.t
  y : V.t
  z0 : V.t
  e0 : EdgeOf G x y
  p : PathOf G y z0
  e : EdgeOf G z0 z
  IHp : ~~ VSet.mem z0 (nodes G p) -> is_simple G (PathOf_add_end p e)
  nmen : is_true (~~ VSet.mem x (nodes G p))
  iss : is_true (is_simple G p)
  Hm : ~ VSet.mem z0 (VSet.add x (nodes G p))
  nmem : VSet.In z0 (nodes G p)
  ============================
   (VSet.E.eq z0 x \/ VSet.In z0 (nodes G p))


DEBUG:Going to execute:
fun t =>
             tryif Ltac.is_var t then fail "no need to abstract a variable"
               else (let x := fresh "x" in
                     set (x := t) in *; try clearbody x)