be careful when you define classes in modules!

gistfile1.rb

class A
  def a
    "a from A"
  end
end

module B; end

class B::A
  def a
    "a from B"
  end
end

class B::C < A
end

module B
  class D < A
  end
end

B::C.new.a # => "a from A"
B::D.new.a # => "a from B"