Goal:
  
  A : Type
  B : Type
  C : Type
  R : A → B → C → Type
  b0 : All3 R [] [] []
  b'' : list A
  b''0 : list B
  b' : list C
  b : All3 R b'' b''0 b'
  ============================
   ({|
      pr1 := {| pr1 := []; pr2 := {| pr1 := []; pr2 := [] |} |}; pr2 := b0
    |} =
    {|
      pr1 := {| pr1 := b''; pr2 := {| pr1 := b''0; pr2 := b' |} |}; pr2 := b
    |}
    → ∀ (H := CoreTactics.block) (e : NoConfusionHom_All3 A B C R [] [] []
                                        All3_nil b0),
        let H0 := CoreTactics.block in
        noConfusionHom_All3_obligation_2 A B C R [] [] [] All3_nil b0
          (noConfusionHom_All3_obligation_1 A B C R [] [] [] All3_nil b0 e) =
        e)


DEBUG:Going to execute:
hnf in id
Level 5: evaluation returns
hnf in id;
 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
where
id' := b
of type
constr
id := b
of type
constr of type tacvalue