Goal:
  
  A : Type
  B : Type
  p : A → B → bool
  b : B
  l' : list B
  h : is_true (forallb2 p [] (b :: l'))
  ============================
   (Forall2 (λ (x : A) (y : B), p x y) [] (b :: l'))


DEBUG:Going to execute:
<coq-core.plugins.ltac::discriminate@0>
DEBUG LTAC Evaluated term: l
DEBUG LTAC Evaluated term: l'