Some modern theories of syntax introduce functional categories in which the head of a phrase is a functional lexical item. Some functional heads in some languages are not pronounced, but are rather covert. For example, in order to explain certain syntactic patterns which correlate with the speech act a sentence performs, some researchers have posited force phrases (ForceP), whose heads are not pronounced in many languages including English. Similarly, many frameworks assume that covert determiners are present in bare noun phrases such as proper names. Another type is the inflectional phrase, where (for example) a finite verb phrase is taken to be the complement of a functional, possibly covert head (denoted INFL) which is supposed to encode the requirements for the verb to inflect – for agreement with its subject (which is the specifier of INFL), for tense and aspect, etc. If these factors are treated separately, then more specific categories may be considered: tense phrase (TP), where the verb phrase is the complement of an abstract "tense" element; aspect phrase; agreement phrase and so on. Further examples of such proposed categories include topic phrase and focus phrase, which are argued to be headed by elements that encode the need for a constituent of the sentence to be marked as the topic or focus. Variation among theories of syntax Theories of syntax differ in what they regard as a phrase. For instance, while most if not all theories of syntax acknowledge the existence of verb phrases (VPs), Phrase structure grammars acknowledge both finite verb phrases and non-finite verb phrases while dependency grammars only acknowledge non-finite verb phrases. The split between these views persists due to conflicting results from the standard empirical diagnostics of phrasehood such as constituency tests. The distinction is illustrated with the following examples: The Republicans may nominate Newt. - Finite VP in bold The Republicans may nominate Newt. - Non-finite VP in bold The syntax trees of this sentence are next: The constituency tree on the left shows the finite verb string may nominate Newt as a constituent; it corresponds to VP1. In contrast, this same string is not shown as a phrase in the dependency tree on the right. However, both trees, take the non-finite VP string nominate Newt to be a constituent. See also Clause Constituent (linguistics) Dependency grammar Finite verb Head (linguistics) Non-finite verb Phrase structure grammar Sentence (linguistics) Syntactic category Verb phrase Phraseme