at
    is convertible to <code>T</code>. In this case there are two ways where
    the two-argument template forms are helpful: one is to provide the
    respective signatures for operator overloading, the second is
    performance.</p>

    <p>With respect to the operator overloading assume <i>e.g.</i> that
    <code>U</code> is <code>int</code>, that <code>T</code> is an
    user-defined unlimited integer type, and that <code>double
    operator-(double, const T&amp;)</code> exists. If one wants to comput