Tree adjoining grammar Guide, Meaning , Facts, Information and Description
Tree adjoining grammar (TAG) is a grammar formalism often used in computational linguistics and natural language processing, defined by Aravind Joshi. Tree adjoining grammars are somewhat similar to context-free grammars, but the elementary unit is the tree rather than the symbol. Whereas context-free grammars have rules for rewriting symbols as strings of other symbols, tree adjoining grammars have rules for "rewriting" the nodes of tress as other trees (see tree (graph theory) and tree data structure). This is done either via the operation of substitution where the root of one tree is inserted at some leaf node of another or the operaton of adjunction where one tree is ajoined into a (possibly) non-leaf node of another tree, effectively splitting that node into a root and foot which are attached respectively at the root and some leaf of the adjoining tree. Tree adjoining grammars are often described as "mildly context-sensitive", meaning that they possess certain properties that make them more powerful (in terms of weak generative capacity) than context-free grammars, but less powerful than context-sensitive grammars as defined in the Chomsky hierarchy. Mildly context-sensitive grammars are (it is conjectured) powerful enough to model the grammars of natural languages while remaining efficiently parseable in the general case.
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