Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  H : ∀ (x : A) (y : B), R x y
  b : B
  l : list B
  e : 0 = #|b :: l|
  ============================
   (Forall2 R [] (b :: l))


DEBUG:Going to execute:
discriminate e
DEBUG LTAC Evaluated term: e
Level 0: Backtrace:

Not a discriminable equality.
Level 0: Backtrace:

Not a discriminable equality.