Goal:
  
  A : Type
  R : A → A → bool
  R_refl : ∀ x : A, R x x
  a : A
  l : list A
  IHl : is_true (forallb2 R l l)
  ============================
   (is_true (true && forallb2 R l l))


DEBUG:Going to execute:
assumption
DEBUG LTAC Evaluated term: R_refl
DEBUG LTAC Evaluated term: R_refl