Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Prop
  a : A
  l : list A
  c : C
  l'' : list C
  f : Forall3 P (a :: l) [] (c :: l'')
  IHl : ∀ (l' : list B) (l'' : list C), Forall3 P l l' l'' → All3 P l l' l''
  ============================
   (All3 P (a :: l) [] (c :: l''))


DEBUG:Going to execute:
constructor
Level 0: Backtrace:

In environment
A : Type
B : Type
C : Type
P : A → B → C → Prop
a : A
l : list A
f : Forall3 P (a :: l) [] []
IHl : ∀ (l' : list B) (l'' : list C), Forall3 P l l' l'' → All3 P l l' l''
Unable to unify
 "All3 ?M2797 (?M2798 :: ?M2801) (?M2799 :: ?M2802) (?M2800 :: ?M2803)"
with "All3 P (a :: l) [] []".
Level 0: Backtrace:

In environment
A : Type
B : Type
C : Type
P : A → B → C → Prop
a : A
l : list A
f : Forall3 P (a :: l) [] []
IHl : ∀ (l' : list B) (l'' : list C), Forall3 P l l' l'' → All3 P l l' l''
Unable to unify
 "All3 ?M2797 (?M2798 :: ?M2801) (?M2799 :: ?M2802) (?M2800 :: ?M2803)"
with "All3 P (a :: l) [] []".