hpricot：Hpricot::Builder#head

everyday Ruby 
>> require "hpricot"
=> true
>> doc = Hpricot('') {}
=> #<Hpricot::Doc>
>> doc.head('')
ArgumentError: block not supplied
	from /usr/local/ruby-1.8.7/lib/ruby/gems/1.8/gems/hpricot-0.8.2/lib/hpricot/builder.rb:159:in `instance_eval'
	from /usr/local/ruby-1.8.7/lib/ruby/gems/1.8/gems/hpricot-0.8.2/lib/hpricot/builder.rb:159:in `head'
	from /usr/local/ruby-1.8.7/lib/ruby/gems/1.8/gems/hpricot-0.8.2/lib/hpricot/builder.rb:19:in `instance_eval'
	from /usr/local/ruby-1.8.7/lib/ruby/gems/1.8/gems/hpricot-0.8.2/lib/hpricot/builder.rb:19:in `build'
	from /usr/local/ruby-1.8.7/lib/ruby/gems/1.8/gems/hpricot-0.8.2/lib/hpricot/builder.rb:121:in `build'
	from /usr/local/ruby-1.8.7/lib/ruby/gems/1.8/gems/hpricot-0.8.2/lib/hpricot/builder.rb:113:in `tag!'
	from /usr/local/ruby-1.8.7/lib/ruby/gems/1.8/gems/hpricot-0.8.2/lib/hpricot/builder.rb:157:in `head'
	from (irb):3
>> doc.head() {}
=> {emptyelem <head>}
>> doc.head('p') {}
TypeError: can't convert Hpricot::ETag into String
   ...

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