Goal:
  
  A : Type
  P : A → Prop
  a : A
  l : list A
  H : ∀ x : A, In x (a :: l) → P x
  IHl : (∀ x : A, In x l → P x) → Forall P l
  ============================
   (a = a ∨ In a l)


DEBUG:Going to execute:
F x2 x1 x0
Level 2: evaluation returns
<coq-core.plugins.tauto::find_cut@0>
where
k := fun id0 id1 X2 => assert X2; [ exact (id0 id1) | clear id0 ]
     where
     flags := 
     of type
     tauto_flags
of type
tacvalue of type tacvalue