 const versions of <code class="literal">operator()</code> can be used
          for a target function object that is const or, if the target function object
          is held by value, the adapter is const - these semantics have nothing to
          do with the const qualification of a member function, which is referring
          to the type of object pointed to by <code class="literal">this</code> which is specified
          with the first element in the sequence passed to the adapter).
        </p>
<p>