docs: private ImmutableList<Fact> description() {
      String description = null;
      boolean descriptionWasDerived = false;
      for (Step step : steps) {
        if (step.isCheckCall()) {
          checkState(description != null);
          if (step.descriptionUpdate == null) {
            description = null;
            descriptionWasDerived = false;
          } else {
            description = verifyNotNull(step.descriptionUpdate.apply(description));
            descriptionWasDerived = true;
          }
          continue;
        }
  
        if (description == null) {
          description =
              firstNonNull(step.subject.internalCustomName(), step.subject.typeDescription());
        }
      }
      return descriptionWasDerived
          ? ImmutableList.of(fact("value of", description))
          : ImmutableList.<Fact>of();
    }