such systems, the rules are of the form a (u1, tart)li(lz, tarz)ll • • • Il(un, tarn), with the
me aning that by rewriting a string xtaxi ... evolve completely independently, hence we can
replace the rule a OIL tart) I Kuz, tarz) II- • • Il(th, tarn) with n rules a (ui, tari),I < i < n,
without changing