def slideshow
    puts SLIDESHOW_START
    yield
    puts SLIDESHOW_END
end

def slide title
    puts "   
"
    puts "    
#{ title }
"
    puts "    
"
    yield
    puts "    
"
    puts "   
"
end

def item text
    puts "     
#{ text.htmlspecialchars }
"
end

SLIDESHOW_START = <
 ...
 
  

END

SLIDESHOW_END = <
 

END

class String
    def htmlspecialchars
        gsub('&', '&').gsub('<', '<').gsub('>', '>')
    end
end

slideshow {
  slide ("first slide") {
    item "a point"
    item "another point"
  }
  slide ("second slide") {
    item "an interesting point"
    item "an uninteresting point"
  }
}