Notes on computational linguistics.pdf - UCLA Department of ...

Stabler - Lx 185/209 2003
5 Trees, and tree manipulation: second idea
5.1 Nodes and leaves in tree structures
(1) The previous section introduced a standard way of representing non-empty ordered trees uses a two
argument term Label/Subtrees. 19
The argument Label is the label of the tree’s root node, and Subtrees is the sequence of that node’s
subtrees.
A tree consisting of a single node (necessarily a leaf node) has an empty sequence of subtrees.
For example, the 3 node tree with root labeled a and leaves labeled b and c is represented by the term
a/[b/[], c/[]]:
b
While this representation is sufficient to represent arbitrary trees, it is useful extend it by treating
phonological forms not as separate terminal nodes, but as a kind of annotation or feature of their
parent nodes. This distinguishes “empty nodes” from leaf nodes with phonological content; only the
latter possess (non-null) phonological forms. Thus in the tree fragment depicted above, the phonological
forms Mary and walks are to be interpreted not as a separate nodes, but rather as components of their
parent DP and V nodes.
(2) While this representation is sufficient to represent arbitrary trees, it is useful extend it by treating
phonological forms not as separate terminal nodes, but as a kind of annotation or feature of their parent
nodes. This distinguishes “empty nodes” from leaf nodes with phonological content; only the latter
possess (non-null) phonological forms. Thus in the tree fragment depicted just below, the phonological
forms Mary and walks are to be interpreted not as a separate nodes, but rather as components of their
parent DP and V nodes.
DP
Mary
VP
a
V
walks
There are a number of ways this treatment of phonological forms could be worked out. For example,
the phonological annotations could be regarded as features and handled with the feature machinery,
perhaps along the lines described in Pollard and Sag (1989, 1993). While this is arguably the formalization
most faithful to linguists’ conceptions, we have chosen to represent trees consisting of a single
19 This notation is discussed more carefully in Stabler (1992, p65).
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c
V’
VP
V’
V