What we see experimentally is when three of those heme groups are filled with oxygen, the structure of the hemoglobin will exist predominantly in the r state, which is exactly what we see according to our concerted model. Based on the concerted model, when three of these heme groups are filled with oxygen, our r state will predominate. So this arrow will be longer than this arrow, which is basically the correct description of what actually happens in nature. But according to this diagram here, if three of these groups are filled with oxygen, our molecule still exists in some intermediate state. It will not exist in the r state. So that is the limitation of the sequential model.