In his classification, the author distinguishes between primary and auxiliary sign languages as well as between single languages and names that are thought to refer to more than one language. The prototype-A class of languages includes all those sign languages that seemingly cannot be derived from any other language. Prototype-R languages are languages that are remotely modelled on a prototype-A language (in many cases thought to have been French Sign Language) by a process Kroeber (1940) called "stimulus diffusion". The families of BSL, DGS, JSL, LSF (and possibly LSG) were the products of creolization and relexification of prototype languages. Creolization is seen as enriching overt morphology in sign languages, as compared to reducing overt morphology in spoken languages.