code: @NotNull
      public OptionalBoolean map(@NotNull BooleanPredicate mapper) {
          if (!isPresent()) {
              return empty();
          }
          Objects.requireNonNull(mapper);
          return OptionalBoolean.of(mapper.test(value));
      }
docs: Invokes the given 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}