function render($diff)
    {
        $xi = $yi = 1;
        $block = false;
        $context = array();

        $nlead = $this->_leading_context_lines;
        $ntrail = $this->_trailing_context_lines;

        $output = $this->_startDiff();

        $diffs = $diff->getDiff();
        foreach ($diffs as $i => $edit) {
            /* If these are unchanged (copied) lines, and we want to keep
             * leading or trailing context lines, extract them from the copy
             * block. */
            if (is_a($edit, 'Text_Diff_Op_copy')) {
                /* Do we have any diff blocks yet? */
                if (is_array($block)) {
                    /* How many lines to keep as context from the copy
                     * block. */
                    $keep = $i == count($diffs) - 1 ? $ntrail : $nlead + $ntrail;
                    if (count($edit->orig) <= $keep) {
                        /* We have less lines in the block than we want for
                         * context => keep the whole block. */
                        $block[] = $edit;
                    } else {
                        if ($ntrail) {
                            /* Create a new block with as many lines as we need
                             * for the trailing context. */
                            $context = array_slice($edit->orig, 0, $ntrail);
                            $block[] = new Text_Diff_Op_copy($context);
                        }
                        /* @todo */
                        $output .= $this->_block($x0, $ntrail + $xi - $x0,
                                                 $y0, $ntrail + $yi - $y0,
                                                 $block);
                        $block = false;
                    }
                }
                /* Keep the copy block as the context for the next block. */
                $context = $edit->orig;
            } else {
                /* Don't we have any diff blocks yet? */
                if (!is_array($block)) {
                    /* Extract context lines from the preceding copy block. */
                    $context = array_slice($context, count($context) - $nlead);
                    $x0 = $xi - count($context);
                    $y0 = $yi - count($context);
                    $block = array();
                    if ($context) {
                        $block[] = new Text_Diff_Op_copy($context);
                    }
                }
                $block[] = $edit;
            }

            if ($edit->orig) {
                $xi += count($edit->orig);
            }
            if ($edit->final) {
                $yi += count($edit->final);
            }
        }

        if (is_array($block)) {
            $output .= $this->_block($x0, $xi - $x0,
                                     $y0, $yi - $y0,
                                     $block);
        }

        return $output . $this->_endDiff();
    }