Goal:
    A : Type  B : Type  R : A → B → Type  x : A  y0 : B  r : R x y0
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in ∑ y' : B, [y0] = [y'] × R x y')


DEBUG:Going to execute:
match goal with
           | |- let _ := CoreTactics.block in ?T =>
                 match T with
                 | context [ CoreTactics.block ] =>
                     change T; CoreTactics.intros_until_block
                 end
           | _ => CoreTactics.unblock_goal
           end