Class templates and friend function templates

I have a class template like this:

template< typename t >
class c;

I'd also like to have an operator == for objects of types, which are different specializations of c:

template< typename t1, typename t2 >
bool operator ==
(
    s< t1 > const&
,    s< t2 > const&
);

In order to implement this operator, I need it to be a friend of both s< t1 > and s< t2 >. For the sake of safety, I'd like no other specialization of c to have this operator as its friend. Is this possible and, if so, how? 

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