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
  ============================
   ({|
      pr1 :=
        {| pr1 := b0 :: bs; pr2 := {| pr1 := hd :: tl; pr2 := hd :: tl' |} |};
      pr2 := OnOne2All_tl P b0 bs hd tl tl' a
    |} =
    {|
      pr1 :=
        {| pr1 := b0 :: bs; pr2 := {| pr1 := hd :: tl; pr2 := hd :: tl' |} |};
      pr2 := OnOne2All_tl P b0 bs hd tl tl' a
    |})


DEBUG:Going to execute:
constructor