hpricot：Hpricot::Traverse#make

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').make
ArgumentError: an Hpricot document must be built from an input source (a String or IO object.)
	from /usr/local/ruby-1.8.7/lib/ruby/gems/1.8/gems/hpricot-0.8.2/lib/hpricot/parse.rb:33:in `scan'
	from /usr/local/ruby-1.8.7/lib/ruby/gems/1.8/gems/hpricot-0.8.2/lib/hpricot/parse.rb:33:in `make'
	from /usr/local/ruby-1.8.7/lib/ruby/gems/1.8/gems/hpricot-0.8.2/lib/hpricot/tag.rb:12:in `make'
	from /usr/local/ruby-1.8.7/lib/ruby/gems/1.8/gems/hpricot-0.8.2/lib/hpricot/traverse.rb:27:in `make'
	from (irb):3
>> doc.at('p').make("")
=> nil
>> doc.at('p').make("foo")
=> ["foo"]
>> doc
=> #<Hpricot::Doc {elem <p> {elem <b> "foo1" </b>} </p>} {elem <p> "foo2" </p>}>
>> doc.at('p').make("<p>foo3</p><p>foo4</p>")
=> [{elem <p> "foo3" </p>}, {elem <p> "foo4" </p>}]
>> doc
=> #<Hpricot::Doc {elem <p> {elem <b> "foo1" </b>} </p>} {elem <p> "foo2" </p>}>
>> doc.make("<p>foo3</p><p>foo4</p>")
=> [{elem <p> "foo3" </p>}, {elem <p> "foo4" </p>}]
>> doc
=> #<Hpricot::Doc {elem <p> {elem <b> "foo1" </b>} </p>} {elem <p> "foo2" </p>}>
>> Hpricot.make("<p>foo3</p><p>foo4</p>")
=> #<Hpricot::Doc {elem <p> "foo3" </p>} {elem <p> "foo4" </p>}>
>> Hpricot.make("<p>foo3</p><p>foo4</p>").children
=> [{elem <p> "foo3" </p>}, {elem <p> "foo4" </p>}]

有効な使用方法は？