docs: public final boolean add(Object key
                      )
      {
          if (Tracing.isAnyTracingEnabled() && trace.isEntryEnabled())
              trace.entry(this, cclass
                          , "add"
                          , "key=" + key + "(Object)"
                              );
  
          boolean found = false;
          java.util.Set subSet = getSubSet(key);
          synchronized (subSet) {
              found = subSet.add(key);
          } // synchronized (subSet).
  
          if (Tracing.isAnyTracingEnabled() && trace.isEntryEnabled())
              trace.exit(this, cclass
                         , "add"
                         , "returns found=" + found + "(boolean)"
                              );
          return found;
      }