Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  b0 : B
  bs : list B
  hd : A
  hd' : A
  tl : list A
  p : P b0 hd hd'
  e0 : #|bs| = #|tl|
  idx3 : list B
  idx4 : list A
  idx2 : list A
  b : OnOne2All P idx3 idx4 idx2
  ============================
   (∀ e : NoConfusion_OnOne2All A B P
            {|
              pr1 :=
                {|
                  pr1 := b0 :: bs;
                  pr2 := {| pr1 := hd :: tl; pr2 := hd' :: tl |}
                |};
              pr2 := OnOne2All_hd P b0 bs hd hd' tl p e0
            |}
            {|
              pr1 := {| pr1 := idx3; pr2 := {| pr1 := idx4; pr2 := idx2 |} |};
              pr2 := b
            |},
      let H := CoreTactics.block in
      noConfusion_OnOne2All_obligation_2 A B P
        {|
          pr1 :=
            {|
              pr1 := b0 :: bs; pr2 := {| pr1 := hd :: tl; pr2 := hd' :: tl |}
            |};
          pr2 := OnOne2All_hd P b0 bs hd hd' tl p e0
        |}
        {|
          pr1 := {| pr1 := idx3; pr2 := {| pr1 := idx4; pr2 := idx2 |} |};
          pr2 := b
        |}
        (noConfusion_OnOne2All_obligation_1 A B P
           {|
             pr1 :=
               {|
                 pr1 := b0 :: bs;
                 pr2 := {| pr1 := hd :: tl; pr2 := hd' :: tl |}
               |};
             pr2 := OnOne2All_hd P b0 bs hd hd' tl p e0
           |}
           {|
             pr1 := {| pr1 := idx3; pr2 := {| pr1 := idx4; pr2 := idx2 |} |};
             pr2 := b
           |} e) = e)


DEBUG:Going to execute:
match goal with
           | |- ?T => change (let _ := CoreTactics.block in T)
           end