The Quran describes itself as "the discernment" (al-furqān), "the mother book" (umm al-kitāb), "the guide" (huda), "the wisdom" (hikmah), "the remembrance" (dhikr) and "the revelation" (tanzīl; something sent down, signifying the descent of an object from a higher place to lower place). What term is unused in the Quran to indicate previous revelations?