ut</span> <span class="special">&lt;&lt;</span> <span class="string">"(x - y) = "</span> <span class="special">&lt;&lt;</span> <span class="identifier">safe_format</span><span class="special">(</span><span class="identifier">x</span> <span class="special">-</span> <span class="identifier">y</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
    <span cl