Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  R' : ∀ (a : A) (b : B), R a b → Type
  x : A
  y : B
  l : list A
  l' : list B
  r : R x y
  rs : All2 R l l'
  r0 : R' x y r
  a0 : All2_dep R' rs
  b0 : All2_dep R' (All2_cons r rs)
  b'' : list A
  b' : sigma (λ ys : list B, All2 R b'' ys)
  b : All2_dep R' (pr2 (pr2 {| pr1 := b''; pr2 := b' |}))
  ============================
   ({|
      pr1 :=
        {|
          pr1 := x :: l; pr2 := {| pr1 := y :: l'; pr2 := All2_cons r rs |}
        |};
      pr2 := b0
    |} = {| pr1 := {| pr1 := b''; pr2 := b' |}; pr2 := b |}
    → let H := CoreTactics.block in
      NoConfusionHom_All2_dep A B R R' (x :: l) (y :: l') 
        (All2_cons r rs) (All2_dep_cons r0 a0) b0
      → let H0 := CoreTactics.block in All2_dep_cons r0 a0 = b0)


DEBUG:Going to execute:
DepElim.decompose_exists xn id