Goal:
  
  A : Type
  B : Type
  R : A → B → Prop
  x : A
  y0 : B
  l' : list B
  H : R x y0
  a1 : sigma (λ _ : list A, list B)
  a2 : (λ index : sigma (λ _ : list A, list B),
          Forall2 R (pr1 index) (pr2 index)) a1
  ============================
   ({| pr1 := []; pr2 := l' |} = a1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in ∃ y' : B, y0 :: l' = [y'] ∧ R x y')


DEBUG:Going to execute:
try unfold signature in id'2