Goal:
    A : Type  B : Type  R : A → B → Type  x : A  y : list B  a : All2 R [x] y
  ============================
   (let H := CoreTactics.block in ∑ y' : B, y = [y'] × R x y')


DEBUG:Going to execute:
fun tac => lazymatch goal with
                      | H:_ |- _ => tac H
                      end