 L4/M4 refer to the fact that we can embed linguistic phrases within other linguistic phrases to create novel and increasingly complex phrases and sentences. This property is known as recursion, and is present in all known natural languages (indeed, Chomsky suggests that perhaps recursion is the only linguistic universal; see Hauser, Chomsky &amp; Fitch 2002; but see Everett 2005). For instance, just as phrases can be embedded in other phrases to form more complex phrases: