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)
  h1 := x :: xs : list A
  h2 := h : (λ index : list A, All (P y) index) h1
  ============================
   (h = h2
    → let H := CoreTactics.block in let H0 := CoreTactics.block in P y x)


DEBUG:Going to execute:
clearbody id'2 id'1
DEBUG LTAC Evaluated term: (eq_refl : h = h2)