// different from each other. If buildKeepingLast() collapsed duplicates, that might end up
      // not being true.
      Pattern pattern = Pattern.compile("Multiple entries with same key: four=(.*) and four=(.*)");
      assertThat(expected).hasMessageThat().matches(pattern);
      Matcher matcher = pattern.matcher(expected.getMessage());
      assertThat(matcher.matches()).isTrue();

               * the other of which creates new Cancellation instances each time it's called? Yet
               * another alternative is to fill in a non-null value for each of the fields no matter
               * what and to just not use it if !GENERATE_CANCELLATION_CAUSES.
               */
              : requireNonNull(
                  mayInterruptIfRunning

    }

    @Override
    public @NonNull T next() {
      T result = valueOrNull;
      valueOrNull = null;
      // We put the common case first, even though it's unlikely to matter if the code is run much:
      // https://shipilev.net/jvm/anatomy-quarks/28-frequency-based-code-layout/
      if (result != null) {
        return result;
      }
      throw new NoSuchElementException();