Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  l : list A
  a : B
  r1 : list B
  r2 : list B
  h : All2 R l (a :: r1 ++ r2)
  ============================
   (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:
fun id =>
             DepElim.generalize_sig id
              ltac:(fun id id' => DepElim.decompose_exists id id')