hpricot：Hpricot::Traverse#traverse_text

everyday Ruby 
>> require "hpricot"
=> true
>> doc = Hpricot('<p><b>foo1</b></p><p>foo2</p>')
=> #<Hpricot::Doc {elem <p> {elem <b> "foo1" </b>} </p>} {elem <p> "foo2" </p>}>
>> doc.at('p').traverse_text {|e| p e}
"foo1"
=> nil
>> doc.search('p').traverse_text {|e| p e}
NoMethodError: undefined method `traverse_text' for #<Hpricot::Elements:0xb797937c>
	from (irb):4
>> doc = Hpricot('<p><b>foo1</b>hoge</p><p>foo2</p>')
=> #<Hpricot::Doc {elem <p> {elem <b> "foo1" </b>} "hoge" </p>} {elem <p> "foo2" </p>}>
>> doc.at('p').traverse_text {|e| p e}
"foo1"
"hoge"
=> nil