Goal:
  
  A : Type
  P : A → A → Type
  Q : A → Type
  HPQ : ∀ x y : A, Q x → P x y → Q y
  hd : A
  tl : list A
  tl' : list A
  X : OnOne2 P tl tl'
  IHX : All Q tl → All Q tl'
  H : All Q (hd :: tl)
  H0 := {| pr1 := hd :: tl; pr2 := H |}
     : sigma (λ index : list A, All Q index)
  H1 := hd :: tl : list A
  H2 := H : (λ index : list A, All Q index) H1
  ============================
   (H = H2
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All Q (hd :: tl'))


DEBUG:Going to execute:
<coq-core.plugins.ltac::clearbody@0> $1