tafunction that evaluates to <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code>
                    if a certain type, <code class="computeroutput"><span class="identifier">S</span></code>
                    is a string, <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span></code>
                    otherwise (See <a href="../../../../../.