def size
      raise Erlang::ImproperListError if improper?
      return 0 if empty?
      size = 0
      list = self
      until list.empty?
        list = list.tail
        size += 1
      end
      return size
    end
def size
      raise Erlang::ImproperListError if improper?
      return 0 if empty?
      size = 0
      list = self
      until list.empty?
        list = list.tail
        size += 1
      end
      return size
    end
Return the number of items in this `Erlang::List`.
 @return [Integer]