> I want to get the operational value of a buffer
> variable.  If the buffer has a local value, one
> cannot use the default-value implementation.
> And if the local value in nil, I have to get the
      ^^^^^^^^^^^^^^^^^^^^^^^^^
> default-value.

You said this in your previous mail, and I meant to
correct it.  It's not at all about the buffer-local
value being `nil'.  `nil' is a legitimate value.

It's about whether the variable _has_ a buffer-local
value.  If it has a buffer-local value of `nil' then
it has a buffer-local value.