Goal:
  
  A : Type
  B : Type
  P : B → A → A → Type
  Q : A → Type
  HPQ : ∀ (i : B) (x y : A), Q x → P i x y → Q y
  b : B
  bs : list B
  hd : A
  hd' : A
  tl : list A
  p : P b hd hd'
  e : #|bs| = #|tl|
  H : All Q (hd :: tl)
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in All Q (hd' :: tl))


DEBUG:Going to execute:
fun id id' id'1 t =>
             try rename id into id',id' into id; cont id'1 id