             This is essentially the same as the <code class="computeroutput"><span class="identifier">functor_input</span></code>
                  policy except that the (user supplied) function object exposes
                  separate <code class="computeroutput"><span class="identifier">unique</span></code>
                  and <code class="computeroutput"><span class="identifier">shared</span></code> sub
                  classes, allowing to integrate the functors <span class="emphasis"><em>unique</