*
     * - a `getFooOrDefault(@Nullable Foo defaultValue)` method that returns
     *   `map.getOrDefault(FOO_KEY, defaultValue)`
     *
     * - a call that consults a chain of maps, as in `mapA.getOrDefault(key, mapB.getOrDefault(key,
     *   ...))`
     *
     * So it makes sense for the parameter (and thus the return type) to be @Nullable.
     *
     * Two other points:
     *

...