Goal:
  
  A : Type
  B : Type
  C : Type
  Q : C → Type
  P : C → A → Prop
  x : A
  xs : list A
  y : C
  X : Q y
  h : All (P y) (x :: xs)
  h0 := {| pr1 := x :: xs; pr2 := h |}
     : sigma (λ index : list A, All (P y) index)
  ============================
   (let H := CoreTactics.block in let H0 := CoreTactics.block in P y x)


DEBUG:Going to execute:
set (id'2 := pr2 id'); set (id'1 := pr1 id') in id'2;
            hnf in (value of id'1), (value of id'2);
            try red in (type of id'2);
            match goal with
            | id'1:=?t:_
              |- _ =>
                  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
            end