  Given the <code class="computeroutput"><span class="identifier">queue</span><span class="special">&lt;</span>
          <span class="identifier">future</span><span class="special">&lt;</span>
          <span class="identifier">T</span> <span class="special">&gt;</span>
          <span class="special">&gt;</span></code> we already constructed for
          <code class="computeroutput"><span class="identifier">wait_first_outcome</span><span class="special">()</span></code>,
          though, we can readily 