This challenge is quite uncommon since it isn't 'algorithmic' nor 'obfuscating'.
You might want to play with this if you enjoy messing around with
singletons. 

[NOTE: read the end of the message!]
     _________________________________________________________________

   Your mission, should you choose to accept it, is to fill in the code
   below so that its execution terminates without error and the output
is
 You won!

module NoOverloading
  def no_redef(*args)
    @no_override ||= []
    args.each { |i| @no_override << i }
  end

  def __no_redef_list__
    supmethods = []
    (ancestors - [self]).each do |i|
      begin
        supmethods += ( i.__no_redef_list__ || [])
      rescue NameError
      end
    end
    (@no_override || []) + supmethods
  end

  def inherited sub
    class << sub
      def method_added(id)
        return if __no_redef_list__.index(id) == nil
        remove_method id
      end
      def alias_method a, b
        # catcha!
      end
      self.freeze
    end
  end
  self.freeze
end

module Unbreakable
class A
  extend NoOverloading
  no_redef :a
  def a; puts "You lost!"; end
end

class Caller
  def self.foo(arg)
    raise "Only classes derived from A!" unless arg.kind_of?
Unbreakable::A
    arg.a
  end
end
Caller.freeze
A.freeze
self.freeze
end

class B < Unbreakable::A
  def a; puts "You won!";       end
  # PUT YOUR CODE HERE!
end

Unbreakable::Caller.foo( B.new )

Details

   You are expected to subclass Unbreakable::A and find a way to override
   method #a. Normally all your code should be in the indicated position.
   Class B must be directly derived from Unbreakable::A. The following is
   not a valid solution:
class B
  def a; puts "You won!"; end
  def kind_of? a
    true
  end
end

  Warning

   This mission is, to the best of my current knowledge, impossible.
     _________________________________________________________________


-- 
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| |__   __ _| |_ ___ _ __ ___   __ _ _ __  
| '_ \ / _` | __/ __| '_ ` _ \ / _` | '_ \ 
| |_) | (_| | |_\__ \ | | | | | (_| | | | |
|_.__/ \__,_|\__|___/_| |_| |_|\__,_|_| |_|
	Running Debian GNU/Linux Sid (unstable)
batsman dot geo at yahoo dot com

...Deep Hack Mode -- that mysterious and frightening state of
consciousness where Mortal Users fear to tread.
	-- Matt Welsh