Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  a : A
  l : list A
  H : ∀ (x : A) (y : B), R x y
  IHl : ∀ l' : list B, #|l| = #|l'| → Forall2 R l l'
  e : S #|l| = #|[]|
  ============================
   (Forall2 R (a :: l) [])


DEBUG:Going to execute:
discriminate e
DEBUG LTAC Evaluated term: e