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)
  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
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in All Q (hd' :: tl))


DEBUG:Going to execute:
generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear id' id;
            try unfold signature in id'2; hnf in id'2; 
            simpl in id'2; rename id'2 into id; cont id id id'1 t