The empty category assumed (under government and binding theory) to be present in the vacant subject position left by pro-dropping is known as pro, or as "little pro" (to distinguish it from "big PRO", an empty category associated with non-finite verb phrases). Cross-linguistic variation It has been observed that pro-drop languages are those with either rich inflection for person and number (Persian, Polish, Portuguese, etc.) or no such inflection at all (Japanese, Chinese, Korean, etc.), but languages that are intermediate (English, French, etc.) are non-pro-drop. While the mechanism by which overt pronouns are more "useful" in English than in Japanese is obscure, and there are exceptions to this observation, it still seems to have considerable descriptive validity. As Huang puts it, "Pro-drop is licensed to occur either where a language has full agreement, or where a language has no agreement, but not where a language has impoverished partial agreement." In pro-drop languages with a highly inflected verbal morphology, the expression of the subject pronoun is considered unnecessary because the verbal inflection indicates the person and number of the subject, thus the referent of the null subject can be inferred from the grammatical inflection on the verb. Barbosa defines these typological patterns as null-subject languages (NSL), expressing that the term itself, pro-drop, can be subcategorized into categories such as: topic (discourse) pro-drop, partial NSL (partial pro-drop) and consistent NSL (full pro-drop). Topic pro-drop languages In everyday speech there are instances when who or what is being referred to — namely, the topic of the sentence — can be inferred from context. Languages which permit the pronoun to be inferred from contextual information are called topic-drop (also known as discourse pro-drop) languages: thus, topic pro-drop languages allow referential pronouns to be omitted, or be phonologically null. (In contrast, languages that lack topic pro-drop as a mechanism would still require the pronoun.)These dropped pronouns can be inferred from previous discourse, from the context of the conversation, or generally shared knowledge. Among major languages, some which might be called topic pro-drop languages are Japanese, Korean, and Mandarin. Topic prominent languages like Korean, Mandarin and Japanese have structures which focus more on topics and comments as opposed to English, a subject-prominent language. It is this topic-first nature that enables the inference of omitted pronouns from discourse. Korean The following example from Jung (2004:719) Korean shows the omission of both pronouns in the subject and object