Goal:
  
  A : Type
  R : A → A → Prop
  a : A
  l : list A
  IHl : Forall2 R l l → Forall (λ x : A, R x x) l
  H : Forall2 R (a :: l) (a :: l)
  x : A
  y : A
  l0 : list A
  l' : list A
  H3 : R a a
  H5 : Forall2 R l l
  H0 : x = a
  H1 : l0 = l
  H2 : y = a
  H4 : l' = l
  ============================
   (R a a)


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