guava-gwt/src-super/com/google/common/collect/super/com/google/common/collect/ImmutableSortedMap.java
@SuppressWarnings("unchecked")
  public static <K, V> ImmutableSortedMap<K, V> of() {
    return new Builder<K, V>(NATURAL_ORDER).build();
  }
@SuppressWarnings("unchecked")
  public static <K, V> ImmutableSortedMap<K, V> of() {
    return new Builder<K, V>(NATURAL_ORDER).build();
  }
Casting to any type is safe because the set will never hold any elements.