prepend_example.rb

# Your code you include somewhere
module MySpecialCodeBlock
  def foo(x)
    puts "Twice x is: #{x*2}"
    super
  end
end

# From a libary
class TheRealClass
  def foo(x)
    puts x
  end
end

# somewhere in the bootstrapping of your code...
TheRealClass.prepend MySpecialCodeBlock

y = TheRealClass.new
y.foo(2)