add_attributes_to

# File lib/rdoc/ri/ri_formatter.rb, line 347
    def add_attributes_to(txt)
      tokens = txt.split(%r{(</?(?:b|code|em|i|tt)>)})
      text = AttributeString.new
      attributes = 0
      tokens.each do |tok|
        case tok
        when %r{^</(\w+)>$} then attributes &= ~(ATTR_MAP[$1]||0)
        when %r{^<(\w+)>$}  then attributes  |= (ATTR_MAP[$1]||0)
        else
          tok.split(//).each {|ch| text << AttrChar.new(ch, attributes)}
        end
      end
      text
    end