Why does “most important const” have to be const?

Question

In http://herbsutter.com/2008/01/01/gotw-88-a-candidate-for-the-most-important-const/ it mentions \"most important const\" where by C++ deliberately specifies that binding a

Answer 1

Consider the following:

int& x = 5;
x = 6;

What should happen if this was allowed? By contrast, if you did

const int& x = 5;

there would be no legal way to modify x.