Goal:
  
  A : Type
  B : Type
  C : Type
  P : A → B → C → Prop
  b : B
  l' : list B
  f : Forall3 P [] (b :: l') []
  ============================
   (All3 P [] (b :: l') [])


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

In environment
A : Type
B : Type
C : Type
P : A → B → C → Prop
c : C
l'' : list C
f : Forall3 P [] [] (c :: l'')
Unable to unify
 "All3 ?M2749 (?M2750 :: ?M2753) (?M2751 :: ?M2754) (?M2752 :: ?M2755)"
with "All3 P [] [] (c :: l'')".
Level 0: Backtrace:

In environment
A : Type
B : Type
C : Type
P : A → B → C → Prop
c : C
l'' : list C
f : Forall3 P [] [] (c :: l'')
Unable to unify
 "All3 ?M2749 (?M2750 :: ?M2753) (?M2751 :: ?M2754) (?M2752 :: ?M2755)"
with "All3 P [] [] (c :: l'')".