code: def first(n = nil)
        return list_head unless n
        raise ArgumentError, 'n cannot be negative' if n.negative?
  
        return [] if n.zero? || empty?
  
        list_head.take n
      end
docs: Access the first n item(s) in the list.
  
   @param  n [Integer] the number of items to return.
   @return [Listable] if n = nil.
   @return [Array<Listable>] if n >= 0.