pan></code>. This
        allows the user to make use of a default parameter even when she wants to
        specify a parameter later in the parameter list. Also, the defaults for the
        corresponding associated types are somewhat complicated, so metaprogramming
        is required to compute them, and <code class="computeroutput"><span class="identifier">use_default</span></code>
        can help to simplify the implementation. Finally, the identity of the <code class="computeroutput"><span class="ide