hpricot：Hpricot::Traverse#bogusetag?

>> require "hpricot"
=> true
>> doc = Hpricot('<p>foo1<p>foo2</p>')
=> #<Hpricot::Doc {elem <p> "foo1" {elem <p> "foo2" </p>}}>
>> doc.at("p").bogusetag?
=> false
>> doc = Hpricot('<br><p>foo1</p><p>foo2</p>')
=> #<Hpricot::Doc {emptyelem <br>} {elem <p> "foo1" </p>} {elem <p> "foo2" </p>}>
>> doc.at("br").bogusetag?
=> false
>> doc = Hpricot('<br><p>foo1</p><p>foo2</p><img src="foo.png">')
=> #<Hpricot::Doc {emptyelem <br>} {elem <p> "foo1" </p>} {elem <p> "foo2" </p>} {emptyelem <img src="foo.png">}>
>> doc.at("img").bogusetag?
=> false

どういう場合に true になるのか不明…