Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  R' : ∀ (a : A) (b : B), R a b → Type
  b0 : All2_dep R' All2_nil
  b'' : list A
  b''0 : list B
  b' : All2 R b'' b''0
  b : All2_dep R' b'
  ============================
   ({|
      pr1 := {| pr1 := []; pr2 := {| pr1 := []; pr2 := All2_nil |} |};
      pr2 := b0
    |} =
    {|
      pr1 := {| pr1 := b''; pr2 := {| pr1 := b''0; pr2 := b' |} |}; pr2 := b
    |}
    → let H := CoreTactics.block in
      NoConfusionHom_All2_dep A B R R' [] [] All2_nil All2_dep_nil b0
      → let H0 := CoreTactics.block in All2_dep_nil = b0)


DEBUG:Going to execute:
DepElim.decompose_exists xn id