modules.rb

require 'rspec'

module Evil
  def defn(method_name)
    class_eval <<-METHOD, __FILE__, __LINE__ + 1
      def #{method_name}
      end
    METHOD
  end
end

class Bob
  extend Evil

  defn :foo
end

class Alice
  extend Evil

  defn :bar
end

describe Alice do
  it { should respond_to :bar }
  it { should_not respond_to :foo }
end

describe Bob do
  it { should respond_to :foo }
  it { should_not respond_to :bar }
end