gistfile1.rb

module B
  def whisper(a)
    puts a.downcase
  end
end

class Class
  include B
  def say(a)
    puts a
  end
end

class A
  say("hi")
  whisper("BYE")
end

# ruby 1.8.* & 1.9.* =>
# hi
# bye 
#
# macruby 0.0.4 =>
# hi
# class.rb:16:in `<class:A>': undefined method `whisper' for A:Class (NoMethodError)
#  from class.rb:14:in `<main>'