Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  b0 : B
  bs : list B
  hd : A
  tl : list A
  tl' : list A
  a : OnOne2All P bs tl tl'
  b : B
  bs0 : list B
  hd0 : A
  tl0 : list A
  tl'0 : list A
  b1 : OnOne2All P bs0 tl0 tl'0
  H : {|
        pr1 := b0;
        pr2 :=
          {|
            pr1 := bs;
            pr2 :=
              {|
                pr1 := hd;
                pr2 := {| pr1 := tl; pr2 := {| pr1 := tl'; pr2 := a |} |}
              |}
          |}
      |} =
      {|
        pr1 := b;
        pr2 :=
          {|
            pr1 := bs0;
            pr2 :=
              {|
                pr1 := hd0;
                pr2 := {| pr1 := tl0; pr2 := {| pr1 := tl'0; pr2 := b1 |} |}
              |}
          |}
      |}
  ============================
   ({|
      pr1 :=
        {| pr1 := b0 :: bs; pr2 := {| pr1 := hd :: tl; pr2 := hd :: tl' |} |};
      pr2 := OnOne2All_tl P b0 bs hd tl tl' a
    |} =
    {|
      pr1 :=
        {|
          pr1 := b :: bs0; pr2 := {| pr1 := hd0 :: tl0; pr2 := hd0 :: tl'0 |}
        |};
      pr2 := OnOne2All_tl P b bs0 hd0 tl0 tl'0 b1
    |})


DEBUG:Going to execute:
set (rhs := y) in *