bind_lens is tested with two lenses: first, plus(5, 0),
then span_tag({ id: 'test' }, times(5, 0)). In each case, the model
starts out as 5, then the view is changed to fifteen, and then the model to
five again. So the view should go through 10, 15, 5, 25, 15, 0; the model will
go through 5, 10, 0, 5, 3, 0.