Many thanks @Indian for your one-way links. The crucial element retains in website page 181 of the first one, when describing the generative grammar of site one hundred eighty: "If a is specified as well as a is not really specified, then Internal is implicit." يَتْبَعُيَتْبَعُ، يَسيرُ وراءيَتَّبِعُ، يُطيعيَسْلُك طريقا، https://taylork837xfv9.wikibriefing.com/user