*
 * 2. The supertype of this class could be `Function<@Nullable A, @Nullable B>`, since
 * Converter.apply (like Converter.convert) is capable of accepting null inputs. However, a
 * supertype of `Function<A, B>` turns out to be massively more useful to callers in practice: They
 * want their output to be non-null in operations like `stream.map(myConverter)`, and we can

 *
 * 2. The supertype of this class could be `Function<@Nullable A, @Nullable B>`, since
 * Converter.apply (like Converter.convert) is capable of accepting null inputs. However, a
 * supertype of `Function<A, B>` turns out to be massively more useful to callers in practice: They
 * want their output to be non-null in operations like `stream.map(myConverter)`, and we can