/* Whether the step has finished initialisation. */
        bool created = false;

    std::atomic_bool created{false}; // debugging

        assert(step_->created != isNew);

    if (!isNew) {
        assert(step->created);
        return step;
    }

    if (!isNew) return step;

        assert(!step->created);
        step->created = true;

        assert(!step_->created);
        step_->created = true;

        assert(step->created);

        assert(step_->created);

        assert(step->created);

        assert(step_->created);

                    if (rdep_->deps.empty() && rdep->created) runnable = true;

                    if (rdep_->deps.empty() && rdep_->created) runnable = true;