code: public static <V> HMap singletonHMap(TypeSafeKey<?, V> key, V value) {
          return emptyHMap().put(key, value);
      }
docs: Static factory method for creating a singleton HMap.
  
  @param key   the only mapped key
  @param value the only mapped value
  @param <V>   the only mapped value type
  @return a singleton HMap