[] メソッド実装例

gongo.rb

class Gongo
  def [](v)
    self.class.[] (v) 
  end 

  def self.[](v)
    v * 3 
  end 
end

puts Gongo['du']

gongo = Gongo.new
puts gongo['du']

#
# % ruby gongo.rb
# dududu
# dududu