default @NonNull U primaryOrMapFallback(final @NonNull Function<V, U> mapFallback) {
        return this.primary().orElseGet(() -> mapFallback.apply(this.fallback().get()));
    }
Extract {@link #primary()}, or if it's not present, map {@link #fallback()} to {@code U}.

@param mapFallback function mapping the fallback type to {@code U}
@return extracted or mapped value
cloud-core/src/main/java/org/incendo/cloud/type/Either.java