code: public T marketplace_getSubCategories(CharSequence category)
      throws FacebookException, IOException {
      return this.callMethod(FacebookMethod.MARKETPLACE_GET_SUBCATEGORIES,
                             new Pair<String, CharSequence>("category", category));
    }
docs: Get the subcategories available for a category.
  @param category a category, e.g. "HOUSING"
  @return a T listing the marketplace sub-categories
  @see <a href="http://wiki.developers.facebook.com/index.php/Marketplace.getSubCategories">
  Developers Wiki: marketplace.getSubCategories</a>