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TAG formalism

The primitive elements of the standard TAG formalism are known as elementary trees. ELEMENTARY TREES are of two types: initial trees and auxiliary trees (see Figure 2.1). In describing natural language, INITIAL TREES are minimal linguistic structures that contain no recursion, i.e. trees containing the phrasal structure of simple sentences, NP's, PP's, and so forth. Initial trees are characterized by the following: 1) all internal nodes are labeled by non-terminals, 2) all leaf nodes are labeled by terminals, or by non-terminal nodes marked for substitution. An initial tree is called an X-type initial tree if its root is labeled with type X.
Figure 2.1: Elementary trees in TAG

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