"identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">contract</span> <span class="special">{</span>
    <span class="keyword">template</span><span class="special">&lt;</span><span class="special">&gt;</span> <span class="comment">// Specialization to not copy old values of type `u`.</span>
    <span class="keyword">struct</span> <span class="identifier">is_old_value_copyable</span><span class="special">&lt;</span><span class="identifier">u<