It has been perceived that languages tend to be organized around grammatical categories such as noun and verb, nominative and accusative, or present and past. The grammar of language is organized around such 41. ______ fundamental categories, though many languages express the relationships between words and syntax in the other discrete ways. 42. ______ In addition to make substantial use of discrete categories, language 43. ______ has the important property that organizes elements into recursive structures; 44. ______ which allows, for example, a noun phrase to contain another noun phrase (as in 45. ______ "the chimpanzee’s lips") or a clause to contain a clause (as in "I think that it’s raining"). Because recursion in grammar was implicitly recognized much earlier, 46. ______ the importance of this aspect of language became more popular after the 1957 publication of Noam Chomsky’s book Syntactic Structures, which presents a 47. ______ formal grammar of a fragment of English. Prior to this, the most detail 48. ______ descriptions of linguistic systems were of phonological or morphological systems. Chomsky used a context-free grammar augment with transformations. 49. ______ Since then, following the trend of Chomskyan linguistics, context-free grammars have written for substantial fragments of various languages. It has been 50. ______ demonstrated, however, that human languages include cross-serial dependencies, which cannot be handled adequately by context-free grammars.