android/guava/src/com/google/common/collect/ImmutableSortedMap.java
public static <K extends Comparable<?>, V> Builder<K, V> naturalOrder() {
    return new Builder<>(Ordering.natural());
  }
public static <K extends Comparable<?>, V> Builder<K, V> naturalOrder() {
    return new Builder<>(Ordering.natural());
  }
Returns a builder that creates immutable sorted maps whose keys are ordered by their natural
ordering. The sorted maps use {@link Ordering#natural()} as the comparator.