code: public static <K, V> SolidMap<K, V> empty() {
          //noinspection unchecked
          return (SolidMap<K, V>) EMPTY;
      }
docs: Returns an empty {@link SolidMap}.
  
  @param <K> a type of keys.
  @param <V> a type of values.
  @return an empty {@link SolidMap}.