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'
  e : #|bs| = #|tl|
  b : B
  bs0 : list B
  hd0 : A
  hd'0 : A
  tl0 : list A
  p0 : P b hd0 hd'0
  e0 : #|bs0| = #|tl0|
  H : {|
        pr1 := b0;
        pr2 :=
          {|
            pr1 := bs;
            pr2 :=
              {|
                pr1 := hd;
                pr2 :=
                  {|
                    pr1 := hd';
                    pr2 := {| pr1 := tl; pr2 := {| pr1 := p; pr2 := e |} |}
                  |}
              |}
          |}
      |} =
      {|
        pr1 := b;
        pr2 :=
          {|
            pr1 := bs0;
            pr2 :=
              {|
                pr1 := hd0;
                pr2 :=
                  {|
                    pr1 := hd'0;
                    pr2 :=
                      {| pr1 := tl0; pr2 := {| pr1 := p0; pr2 := e0 |} |}
                  |}
              |}
          |}
      |}
  ============================
   ({|
      pr1 :=
        {| pr1 := b0 :: bs; pr2 := {| pr1 := hd :: tl; pr2 := hd' :: tl |} |};
      pr2 := OnOne2All_hd P b0 bs hd hd' tl p e
    |} =
    {|
      pr1 :=
        {|
          pr1 := b :: bs0; pr2 := {| pr1 := hd0 :: tl0; pr2 := hd'0 :: tl0 |}
        |};
      pr2 := OnOne2All_hd P b bs0 hd0 hd'0 tl0 p0 e0
    |})


DEBUG:Going to execute:
let rhs := fresh in
           set (rhs := y) in *; pattern sigma rhs; clearbody rhs; destruct H;
            simpl
DEBUG LTAC Evaluated term: H
DEBUG LTAC Evaluated term: ({|
                              pr1 := b0;
                              pr2 :=
                                {|
                                  pr1 := bs;
                                  pr2 :=
                                    {|
                                      pr1 := hd;
                                      pr2 :=
                                        {|
                                          pr1 := hd';
                                          pr2 :=
                                            {|
                                              pr1 := tl;
                                              pr2 := {| pr1 := p; pr2 := e |}
                                            |}
                                        |}
                                    |}
                                |}
                            |} =
                            {|
                              pr1 := b;
                              pr2 :=
                                {|
                                  pr1 := bs0;
                                  pr2 :=
                                    {|
                                      pr1 := hd0;
                                      pr2 :=
                                        {|
                                          pr1 := hd'0;
                                          pr2 :=
                                            {|
                                              pr1 := tl0;
                                              pr2 :=
                                                {| pr1 := p0; pr2 := e0 |}
                                            |}
                                        |}
                                    |}
                                |}
                            |})
Level 6: type of H
 has value 
({|
   pr1 := b0;
   pr2 :=
     {|
       pr1 := bs;
       pr2 :=
         {|
           pr1 := hd;
           pr2 :=
             {|
               pr1 := hd';
               pr2 := {| pr1 := tl; pr2 := {| pr1 := p; pr2 := e |} |}
             |}
         |}
     |}
 |} =
 {|
   pr1 := b;
   pr2 :=
     {|
       pr1 := bs0;
       pr2 :=
         {|
           pr1 := hd0;
           pr2 :=
             {|
               pr1 := hd'0;
               pr2 := {| pr1 := tl0; pr2 := {| pr1 := p0; pr2 := e0 |} |}
             |}
         |}
     |}
 |})