code: @NotNull
      public OptionalBoolean mapToBoolean(@NotNull ToBooleanFunction<? super T> mapper) {
          if (!isPresent()) return OptionalBoolean.empty();
          return OptionalBoolean.of(mapper.applyAsBoolean(value));
      }
docs: Invokes mapping function on inner value if present.
  
  @param mapper  mapping function
  @return an {@code OptionalBoolean} with transformed value if present,
  otherwise an empty {@code OptionalBoolean}
  @throws NullPointerException if value is present and
  {@code mapper} is {@code null}