Goal:
  
  A : Type
  B : Type
  C : Type
  R : A → C → Type
  g : A → B → C
  x : A
  xs : list A
  ih : ∀ l' : list B,
         All2 (λ (x : A) (y : B), R x (g x y)) xs l'
         → All2 R xs (map2 g xs l')
  y : B
  ys : list B
  z : All2 (λ (x : A) (y : B), R x (g x y)) (x :: xs) (y :: ys)
  ============================
   (All2 R (x :: xs) (g x y :: map2 g xs ys))


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

In environment
A : Type
B : Type
C : Type
R : A → C → Type
g : A → B → C
x : A
xs : list A
ih : ∀ l' : list B,
       All2 (λ (x : A) (y : B), R x (g x y)) xs l' → All2 R xs (map2 g xs l')
z : All2 (λ (x : A) (y : B), R x (g x y)) (x :: xs) []
Unable to unify "All2 ?M2976 (?M2977 :: ?M2979) (?M2978 :: ?M2980)" with
 "All2 R (x :: xs) []".
Level 0: Backtrace:

In environment
A : Type
B : Type
C : Type
R : A → C → Type
g : A → B → C
x : A
xs : list A
ih : ∀ l' : list B,
       All2 (λ (x : A) (y : B), R x (g x y)) xs l' → All2 R xs (map2 g xs l')
z : All2 (λ (x : A) (y : B), R x (g x y)) (x :: xs) []
Unable to unify "All2 ?M2976 (?M2977 :: ?M2979) (?M2978 :: ?M2980)" with
 "All2 R (x :: xs) []".