Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  x : A
  y : list B
  a : Forall2 R [x] y
  a0 := {| pr1 := {| pr1 := [x]; pr2 := y |}; pr2 := a |}
     : sigma
         (λ index : sigma (λ _ : list A, list B),
            Forall2 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),
          Forall2 R (pr1 index) (pr2 index)) a1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in ∃ y' : B, y = [y'] ∧ R x y')


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