hpricot：Hpricot::Builder#add_child

everyday Ruby 
>> require "hpricot"
=> true
>> doc = Hpricot('')
=> #<Hpricot::Doc>
>> doc.add_child(Hpricot::Elem.new("p"))
NoMethodError: undefined method `add_child' for #<Hpricot::Doc>
	from (irb):3
>> doc = Hpricot('') {}
=> #<Hpricot::Doc>
>> doc.add_child(Hpricot::Elem.new("p"))
=> {emptyelem <p>}
>> doc
=> #<Hpricot::Doc {emptyelem <p>}>