def find(text)
      nodes = parser.parse(text)
      nodes.length.times.find do |index|
        if (song = Song.new(nodes[index..-1], rule: @rule)).valid?
          break song
        end
      end
    end
def find(text)
      nodes = parser.parse(text)
      nodes.length.times.find do |index|
        if (song = Song.new(nodes[index..-1], rule: @rule)).valid?
          break song
        end
      end
    end
Find one valid song from given text.
 @return [Ikku::Song]