def next_value(val)
      index = index_of(val)
      return nil unless index
      return nil if index == (get_length - 1)
      return get(index + 1)
    end
def next_value(val)
      index = index_of(val)
      return nil unless index
      return nil if index == (get_length - 1)
      return get(index + 1)
    end
return the next of given. Nil if item not in list or there is not next