* behavior, so it knows that `convert(a)` returns a non-nullable value, and we don't need to
     * perform even a cast, much less a runtime check.
     *
     * All that said, don't forget that everyone should call converter.convert() instead of
     * converter.apply(), anyway. If clients use only converter.convert(), then their nullness
     * checkers are unlikely to ever look at the annotations on this declaration.

     * behavior, so it knows that `convert(a)` returns a non-nullable value, and we don't need to
     * perform even a cast, much less a runtime check.
     *
     * All that said, don't forget that everyone should call converter.convert() instead of
     * converter.apply(), anyway. If clients use only converter.convert(), then their nullness
     * checkers are unlikely to ever look at the annotations on this declaration.