ze <code class="computeroutput"><span class="identifier">proto</span><span class="special">::</span><span class="identifier">deep_copy</span></code>
          as the expression contains literals (the <code class="computeroutput"><span class="char">'{'</span></code>,
          <code class="computeroutput"><span class="char">','</span></code>, and <code class="computeroutput"><span class="char">'}'</span></code>)
          which normally get embedded in the proto expression by reference only.
          The de