hpricot：Hpricot::Elements#filter

everyday Ruby 
>> require "hpricot"
=> true
>> doc = Hpricot('<p>foo1</p><p>foo2</p>')
=> #<Hpricot::Doc {elem <p> "foo1" </p>} {elem <p> "foo2" </p>}>
>> (doc / "p").filter("p")
=> #<Hpricot::Elements[{elem <p> "foo1" </p>}, {elem <p> "foo2" </p>}]>
>> (doc / "p").filter("a")
=> #<Hpricot::Elements[]>

まともな使い方を思いつかなかった…