features a discussion on whether retroflexion is valid in particular cases. The Ṛg-veda is a collection of books, created by multiple authors from distant parts of ancient India. These authors represented different generations, and the mandalas 2 to 7 are the oldest while the mandalas 1 and 10 are relatively the youngest. Yet, the Vedic Sanskrit in these books of the Ṛg-veda "hardly presents any dialectical diversity", states Louis Renou – an Indologist known for his scholarship of the Sanskrit literature and the Ṛg-veda in particular. According to Renou, this implies that the Vedic Sanskrit language had a "set linguistic pattern" by the second half of the 2nd millennium BCE. Beyond the Ṛg-veda, the ancient literature in Vedic Sanskrit that has survived into the modern age include the Samaveda, Yajurveda, Atharvaveda, along with the embedded and layered Vedic texts such as the Brahmanas, Aranyakas, and the early Upanishads. These Vedic documents reflect the dialects of Sanskrit found in the various parts of the northwestern, northern, and eastern Indian subcontinent. Vedic Sanskrit was both a spoken and literary language of ancient India. According to Michael Witzel, Vedic Sanskrit was a spoken language of the semi-nomadic Aryans who temporarily settled in one place, maintained cattle herds, practiced limited agriculture, and after some time moved by wagon trains they called . The Vedic Sanskrit language or a closely related Indo-European variant was recognized beyond ancient India as evidenced by the "Mitanni Treaty" between the ancient Hittite and Mitanni people, carved into a rock, in a region that now includes parts of Syria and Turkey. Parts of this treaty, such as the names of the Mitanni princes and technical terms related to horse training, for reasons not understood, are in early forms of Vedic Sanskrit. The treaty also invokes the gods Varuna, Mitra, Indra, and Nasatya found in the earliest layers of the Vedic literature. The Vedic Sanskrit found in the Ṛg-veda is distinctly more archaic than other Vedic texts, and in many respects, the Rigvedic language is notably more similar to those found in the archaic texts of Old Avestan Zoroastrian Gathas and Homer's Iliad and Odyssey. According to Stephanie W. Jamison and Joel P. Brereton – Indologists known for their translation of the Ṛg-veda – the Vedic Sanskrit literature "clearly inherited" from Indo-Iranian and Indo-European times the social structures such as the role of the poet and the priests, the patronage economy,