docs: public static String upstreamToString(Channel upstream) {
          if (upstream == null) {
              linkedRemaining.set(null);
              return "";
          }
          if (linkedRemaining.get() == null) {
              linkedRemaining.set(1);
          }
          if (linkedRemaining.get() <= 0) {
              linkedRemaining.set(null);
              return "↔…";
          }
          linkedRemaining.set(linkedRemaining.get() - 1);
  
          // Build continuation.
          StringBuilder builder = new StringBuilder();
          builder.append('↔')
              .append(Channel.toString(upstream));
          linkedRemaining.set(null);
          return builder.toString();
  
      }