Among Hassaniya Arabic speakers in southern Morocco and Mauritania, states Sean Hanretta – a professor of African History, the term "Bidan" is a "caste synecdoche" that refers to "Hassani" (warrior) and "Zwaya" (clerical) clans. In the slave castes, they recognized two layers, the "`Abid" (slaves) and "Haratins" (freed slaves). According to Remco Ensel – a professor of Anthropology specializing in Maghreb studies, the word "Haratin" in Moroccan is a pejorative that connotes "subordination, disrepute" and in contemporary literature, it is often replaced with "Drawi", "Drawa", "Sahrawi", "Sahrawa" or other regional terms. The "Haratins" historically lived segregated from the main society, in a rural isolation. Their subjugation regardless of their religion was sometimes ideologically justified by nobles and some Islamic scholars, even though some scholars took a more nuanced view that Muslims can only enslave non-Muslims and they should not enslave other Muslims, states Hamel – a professor of History specializing in African Studies. They along with "Swasin" in Morocco and other northern fringe societies of the Sahara, were a part of a social hierarchy that included the upper strata of nobles, religious specialists and literati, followed by freemen, nomadic pastoral strata and slaves. The Haratin were hierarchically higher than the "`Abid" (descendant of slaves) at the very bottom, but lower than "Ahrar". This hierarchy, states Ensel, has been variously described as ethnic groups, estates, quasi-castes, castes or classes.