Goal:
  
  A : Type
  B : Type
  C : Type
  D : Type
  R : C → D → Type
  f : A → C
  g : B → D
  a : A
  l : list A
  IHl : ∀ l' : list B,
          All2 R (map f l) (map g l')
          → All2 (λ (x : A) (y : B), R (f x) (g y)) l l'
  H1 : sigma (λ _ : list C, list D)
  H : All2 R (pr1 H1) (pr2 H1)
  ============================
   ({| pr1 := f a :: map f l; pr2 := [] |} = H1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      All2 (λ (x : A) (y : B), R (f x) (g y)) (a :: l) [])


DEBUG:Going to execute:
hnf in id;
            match type of id with
            | sigma _ =>
                let xn := fresh id "'" in
                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'
            | _ =>
                cbv beta iota delta [TODO projection TODO projection]
                 in id, id'
            end