code: public void setMode(final String mode) {
          this.mode = Strings.toLowerCase(mode);
          if (mode == null || !MODES.contains(mode)) {
              throw new IllegalArgumentException("Invalid mode [" + mode + "]");
          }
      }
docs: Sets the content mode (Atom 0.3 only).
  <p>
  The mode indicates how the value was/will-be encoded in the XML feed.
  <p>
  
  @param mode the content mode, <b>null</b> if none.