/span><span class="identifier">C</span><span class="special">,</span><span class="identifier">P</span><span class="special">&gt;</span></code> hold a reference to a callable Python
          object which, when invoked from Python, expects a single argument c convertible
          to C and creates a Python iterator that traverses <code class="computeroutput"><span class="special">[</span><span class="identifier">c</span><span class="special">.</span><span class="identifier">begin</span><span class="special">