Access Issues

Hi, everyone. I'd like to suggest the following code:

module test1
    implicit none

    type :: a
        integer :: sa
    end type a

end module test1

module test2
    use test1
    implicit none

    type,extends(a) :: b
        private
        integer :: sb
    end type b

end module test2

program main
    use test2
    implicit none

    type(b) :: var
    write(*,*) var%sa    ! sa can be accessed.
    write(*,*) var%sb    ! sb is blocked.

end program main

Here comes the question: How can I block sa to be seen from the outside of the type defination module test2 ? In other words how can sa made to be a private variable within type b ? Appriciate any ideas and many thanks.