this(out, null);
    }

    public SystemLogger(@Nullable OutputStream out, @Nullable Level threshold) {
        this.out = new PrintWriter(toPsOrDef(out, System.err), true);
        this.threshold = Objects.requireNonNullElse(threshold, Level.INFO);
    }

    private PrintStream toPsOrDef(OutputStream outputStream, PrintStream def) {
        if (outputStream == null) {
            return def;
        }

   * lazy evaluation of the fallback instance, using {@link Optional#or(Supplier)
   * first.or(supplier)}.
   *
   * <p><b>Java 9 users:</b> use {@code java.util.Objects.requireNonNullElse(first, second)}
   * instead.
   *
   * @return {@code first} if it is non-null; otherwise {@code second} if it is non-null
   * @throws NullPointerException if both {@code first} and {@code second} are null