Anyone figured out how package checksums are calculated?

I have a package, built in 4.01 that after a Build, exports what looks
like valid package, but on import, reports a Checksum error.

(Got to see a new icon, a little red box, white X in it, at the package
level in the catalog. Alas, it does NOT say which object had the
checksum issue. (unless it means the overall package checksum)).

I want to see if the checksum is really wrong, or what else might
possibly have gone wrong.

I have tried importing to 4.02 latest builds and still no joy.