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')
  z1 : sigma (λ _ : list A, list B)
  z : All2 (λ (x : A) (y : B), R x (g x y)) (pr1 z1) (pr2 z1)
  ============================
   ({| pr1 := x :: xs; pr2 := [] |} = z1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All2 R (x :: xs) [])


DEBUG:Going to execute:
destruct id as [xn id]; DepElim.decompose_exists xn id;
            cbv beta iota delta [TODO projection TODO projection] in id, id';
            DepElim.decompose_exists id id'