A question regarding exception specification

[LarryChen]

If a class inherits from another and overrides a virtual function, then the exception-specification for the overridden function must be at least as restrictive. What does it mean? Here is an example,

class B
{
public:
	virtual void foo() throw (int)
	{
	}
};

class D: public B
{
public:
	virtual void foo() throw (double)
	{
	}
};

It compiles fine. In the derived class, virtual function foo throws double and in the base class, foo throws int. So double is supposed to be at least as restrictive? Thanks.