Goal:
  
  A : Type
  B : Type
  P : A → B → Prop
  a : A
  l : list A
  b : B
  l' : list B
  f : Forall2 P (a :: l) (b :: l')
  IHl : ∀ l' : list B, Forall2 P l l' → All2 P l l'
  ============================
   (All2 P (a :: l) (b :: l'))


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

In environment
A : Type
B : Type
P : A → B → Prop
a : A
l : list A
f : Forall2 P (a :: l) []
IHl : ∀ l' : list B, Forall2 P l l' → All2 P l l'
Unable to unify "All2 ?M2754 (?M2755 :: ?M2757) (?M2756 :: ?M2758)" with
 "All2 P (a :: l) []".
Level 0: Backtrace:

In environment
A : Type
B : Type
P : A → B → Prop
a : A
l : list A
f : Forall2 P (a :: l) []
IHl : ∀ l' : list B, Forall2 P l l' → All2 P l l'
Unable to unify "All2 ?M2754 (?M2755 :: ?M2757) (?M2756 :: ?M2758)" with
 "All2 P (a :: l) []".