docs: public static String join(Iterable self, String separator) {
          StringBuilder buffer = new StringBuilder();
          boolean first = true;
  
          if (separator == null) separator = "";
  
          for (Object value : self) {
              if (first) {
                  first = false;
              } else {
                  buffer.append(separator);
              }
              buffer.append(InvokerHelper.toString(value));
          }
          return buffer.toString();
      }