Lets say operator[] returns a pointer.
For a const map it returns a nullptr.
Seems good so far?
But we want *map[10] = "Hello" to work.
So for non-const map, map[10] must return
a valid pointer even if key 10 doesnt exist
yet.
But this means the semantics of [] depends
on the const-ness of the map: it creates
a key if the map is non-const and does not
if its const. Even if we don't use the result
of [] for an assignment:

	if ( *map[10] == "ten" ){ // creates key 10
		// or not?