 MPI collectives that require a user-specified operation
        (e.g., <code class="computeroutput"><span class="identifier">reduce</span></code> and <code class="computeroutput"><span class="identifier">scan</span></code>), the operation can be an arbitrary
        Python function. For instance, one could concatenate strings with <code class="computeroutput"><span class="identifier">all_reduce</span></code>:
      </p>
<pre class="programlisting"><span class="identifier">mpi</span><span class="special">.<