Goal:
  
  A : Type
  l3 : list A
  R1 : A → A → Type
  R2 : A → A → Type
  R3 : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  o : OnOne2 R1 tl tl'
  IHo : ∀ l3 : list A,
          All2 R2 tl l3
          → (∀ x y : A, R2 x y → R3 x y)
            → (∀ x y z : A, R1 x y → R2 x z → R3 y z) → All2 R3 tl' l3
  H1' : list A
  H1 : list A
  H : All2 R2 (pr1 {| pr1 := H1'; pr2 := H1 |})
        (pr2 {| pr1 := H1'; pr2 := H1 |})
  ============================
   ({| pr1 := hd :: tl; pr2 := l3 |} = {| pr1 := H1'; pr2 := H1 |}
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      (∀ x y : A, R2 x y → R3 x y)
      → (∀ x y z : A, R1 x y → R2 x z → R3 y z) → All2 R3 (hd :: tl') l3)


DEBUG:Going to execute:
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