the mid-2000s, several of the developers of BCG, including Charles Fillmore, Paul Kay, Ivan Sag and Laura Michaelis, collaborated in an effort to improve the formal rigor of BCG and clarify its representational conventions. The result was Sign Based Construction Grammar (SBCG). SBCG is based on a multiple-inheritance hierarchy of typed feature structures. The most important type of feature structure in SBCG is the sign, with subtypes word, lexeme and phrase. The inclusion of phrase within the canon of signs marks a major departure from traditional syntactic thinking. In SBCG, phrasal signs are licensed by correspondence to the mother of some licit construct of the grammar. A construct is a local tree with signs at its nodes. Combinatorial constructions define classes of constructs. Lexical class constructions describe combinatoric and other properties common to a group of lexemes. Combinatorial constructions include both inflectional and derivational constructions. SBCG is both formal and generative; while cognitive-functional grammarians have often opposed their standards and practices to those of formal, generative grammarians, there is in fact no incompatibility between a formal, generative approach and a rich, broad-coverage, functionally based grammar. It simply happens that many formal, generative theories are descriptively inadequate grammars. SBCG is generative in a way that prevailing syntax-centered theories are not: its mechanisms are intended to represent all of the patterns of a given language, including idiomatic ones; there is no 'core' grammar in SBCG. SBCG a licensing-based theory, as opposed to one that freely generates syntactic combinations and uses general principles to bar illicit ones: a word, lexeme or phrase is well formed if and only if it is described by a lexeme or construction. Recent SBCG works have expanded on the lexicalist model of idiomatically combining expressions sketched out in Sag 2012. Goldbergian/Lakovian construction grammar The type of construction grammar associated with linguists like Goldberg and Lakoff looks mainly at the external relations of constructions and the structure of constructional networks. In terms of form and function, this type of construction grammar puts psychological plausibility as its highest desideratum. It emphasizes experimental results and parallels with general cognitive psychology. It also draws on certain principles of cognitive linguistics. In the Goldbergian strand, constructions interact with each other in a network via four inheritance relations: polysemy link, subpart link, metaphorical extension, and finally instance link. Cognitive grammar Sometimes, Ronald Langacker's cognitive grammar framework is described as a type of construction grammar.