ability
                      of an attribute. The output generated by <code class="computeroutput"><span class="identifier">a</span></code>
                      is interspersed with the output generated by <code class="computeroutput"><span class="identifier">b</span></code>. The list generator succeeds
                      if its first embedded generator has been successfully executed
                      at least once (unless the underlying output stream reports
                      an error).
      