class RSpec::Mocks::AnyInstance::Recorder

def observe!(method_name)
  stop_observing!(method_name) if already_observing?(method_name)
  @observed_methods << method_name
  backup_method!(method_name)
  @klass.class_eval(<<-EOM, __FILE__, __LINE__)
    def #{method_name}(*args, &blk)
      self.class.__recorder.playback!(self, :#{method_name})
      self.send(:#{method_name}, *args, &blk)
    end
  EOM
end