Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  x : A
  y : list B
  a : All2 R [x] y
  a0 := {| pr1 := {| pr1 := [x]; pr2 := y |}; pr2 := a |}
     : sigma
         (λ index : sigma (λ _ : list A, list B),
            All2 R (pr1 index) (pr2 index))
  a1 := {| pr1 := [x]; pr2 := y |} : sigma (λ _ : list A, list B)
  a2 := a
     : (λ index : sigma (λ _ : list A, list B),
          All2 R (pr1 index) (pr2 index)) a1
  ============================
   (a = a2
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in ∑ y' : B, y = [y'] × R x y')


DEBUG:Going to execute:
clearbody id'2 id'1
DEBUG LTAC Evaluated term: (eq_refl : a = a2)