 the second element of the fusion sequence has to be compatible with the attribute
        of the second <code class="computeroutput"><span class="identifier">double_</span></code>. If
        we assume to have an instance of a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="specia