Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  l : list A
  a : B
  r1 : list B
  r2 : list B
  h1 : sigma (λ _ : list A, list B)
  h : All2 R (pr1 h1) (pr2 h1)
  ============================
   ({| pr1 := l; pr2 := a :: r1 ++ r2 |} = h1
    → let H := CoreTactics.block in
      (∀ (l : list A) (r2 : list B),
         All2 R l (r1 ++ r2)
         → ∑ l1 l2 : list A, l = l1 ++ l2 × All2 R l1 r1 × All2 R l2 r2)
      → let H0 := CoreTactics.block in
        ∑ l1 l2 : list A, l = l1 ++ l2 × All2 R l1 (a :: r1) × All2 R l2 r2)


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'