Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  x : A
  y : list B
  x0 : A
  y0 : B
  l : list A
  l' : list B
  r : R x0 y0
  a : All2 R l l'
  ============================
   ({| pr1 := [x]; pr2 := y |} = {| pr1 := x0 :: l; pr2 := y0 :: l' |}
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in ∑ y' : B, y = [y'] × R x y')


DEBUG:Going to execute:
simplify * || simplify ?