Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Prop
  p : A → B → C → bool
  a : A
  l : list A
  b : B
  l' : list B
  c : C
  l'' : list C
  Hp : ∀ (x : A) (y : B) (z : C), reflect (P x y z) (p x y z)
  IHl : ∀ (l' : list B) (l'' : list C),
          forallb3 p l l' l'' → Forall3 P l l' l''
  pa : is_true false
  pl : is_true (forallb3 p l l' l'')
  n : ¬ P a b c
  ============================
   (P a b c)


DEBUG:Going to execute:
symmetry; trivial
Level 1: Backtrace:

 The relation (P a) is not a declared reflexive relation. Maybe you need to require the Coq.Classes.RelationClasses library
Level 0: Backtrace:

 The relation (P a) is not a declared reflexive relation. Maybe you need to require the Coq.Classes.RelationClasses library