docs: @Nonnull
      public static String join(@Nonnull Collection<?> strings, @Nonnull String separator) {
          StringBuilder buf = new StringBuilder();
          boolean first=true;
          for (Object s : strings) {
              if(first)   first=false;
              else        buf.append(separator);
              buf.append(s);
          }
          return buf.toString();
      }