prefix in m- is the most widely attested affix in AA that is used to derive nouns. It forms agent nouns, place nouns, and instrument nouns. In some branches, it can also derive abstract nouns and participles. Christopher Ehret has argued that this prefix is a later development that was not present in Proto-Afro-Asiatic, but rather derived from a PAA indefinite pronoun *m-. Such an etymology is rejected by A. Zaborski and Gábor Takács, the latter of whom argues for a PAA *ma- that unites all or some of the meanings in the modern languages. Consonantal root structures and verbal forms A widely attested feature in AA languages is a consonantal structure into which various vocalic "templates" are placed. This structure is particularly visible in the verbs, and is particularly noticeable in the triliteral Semitic verb. Besides for Semitic, vocalic templates are well attested for Cushitic and Berber, where, along with Chadic, it is less productive; it is absent in Omotic. For Egyptian, evidence for the root-and-template structure exists from Coptic. In Semitic, Egyptian, Berber, and some Chadic languages, verbs have no inherent vowels at all; the vowels found in a given verb are dependent on the vocalic template. The degree to which the AA verbal root was triliteral (having three consonants) is debated. It may have originally mostly biconsonantal, to which various affixes (verbal extensions) were then added and lexicalized. As part of these templates, the alternation (apophony) between high vowels (e.g. i, u) and a low vowel (a) in verbal forms is usually described as one of the main characteristics of AA languages: this change codes a variety of different functions. It is unclear whether this system is a common AA trait; the Chadic examples, for instance, show signs of originally deriving from affixes, which could explain the origins of the alterations in other languages as well. There is no agreement about which tenses or aspects Proto-Afroasiatic might have had. Most grammars of AA posit a distinction between perfective and imperfective verbal aspects, which can be found in Cushitic, Berber, Semitic, most Chadic languages, and some Omotic languages. The Egyptian verbal system diverges greatly from that found in the other branches. Verbal extensions Many AA languages use prefixes or suffixes (verbal extensions) to encode various pieces of information about the verb. Three derivational prefixes can be reconstructed for Proto-Afroasiatic: *s- 'causative', *t- 'middle voice' or 'reflexive', and *n-