Mysterious BCs

There are a list of BC (Unchecked / unconfirmed cast) that seem to
appear / disappear for no reasons at all and points to code which
seems to be type-safe by Generics.

I have been trying to reproduce the phenomenon with a simpler test
case with no luck, so I've attached a stripped down version of my code
to illustrate the problem.

To reproduce the problem:

1) Compile the code. FindBugs will report a list of BC's which points
to RationalNumber AND WholeNumber.

2) Under "symbol.arithmetics.scalars", remove RationalNumber.java and
"rationals" package then compile the code. Now all the BC's will
disappear!

Tested on FindBugs 0.9.5; compiled and reproducible with both Mustang
build 70 and Tiger Update 6.

Thanks,
Alex.