code: @NotNull
      public OptionalInt map(@NotNull IntUnaryOperator mapper) {
          if (!isPresent()) return empty();
          return OptionalInt.of(mapper.applyAsInt(value));
      }
docs: Invokes mapping function on inner value if present.
  
  @param mapper  mapping function
  @return an {@code OptionalInt} with transformed value if present,
  otherwise an empty {@code OptionalInt}
  @throws NullPointerException if value is present and
  {@code mapper} is {@code null}
  @since 1.1.3