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In the "Functions" chapter, you give the following example :

int move_player (lua_Stack * ls)
	const int xoffset = lua_tointeger(ls, 1);
	const int yoffset = lua_tointeger(ls, 2);
	lua_settop(ls, 0);       // <-- I am talking about this line
	const Point p = player->move(xoffset, yoffset);
	lua_pushinteger(ls, p.x);
	lua_pushinteger(ls, p.y);
	return 2;

You say it is good practice to maintain a nice clean stack and
recommend to use lua_settop(0). Well certainly not in such a
Lua API is designed so that it is useless to pop arguments from the
stack: the return value already tells how many results there are
inside the stack.
The interpreter will automatically perform the equivalent of
lua_settop(0) after the call.
Worse: in some situations, it can yield to weird bugs.
If an argument is retrieved with lua_tostring, flushing the stack
could make the garbage collector to delete the content of the string,
yielding the const char* argument pointing to invalid memory!