'That' is the question

As a linguist, I was delighted to read about Bernard Lamb's campaign to raise the standard of students' English ("Gobbledegook and ghastly grammar cast a murky spell on coherence", 2 September). But "can easily sell" is a defining clause, so shouldn't the last line of the limerick read: "And write books that can easily sell"?

Janet Fraser, Senior lecturer in translation, University of Westminster.