Nonterminals are underspecified nodes used in templates to represent entire semantic subgraphs. They can correspond directly to a particular node in the source DMRS, but in general they summarise overall properties which can be distributed across the subgraph.
Properties of chunks are mostly determined by their central nodes (§2.3.3), but the chunk centre itself does not have to participate in the functional graph if it has a scopal operator. When present, the operator is the top node of the chunk subgraph and serves as the hook in the *MRS composition system (§2.3.2), but unlike the central node, it has a limited impact on the chunk properties. For instance, an extra adverb in a subordinate clause should not affect the form of a template in most scenarios.
Whenever one of the chunk centres does not participate directly in a functional graph, we identify its closest explicitly present scopal parent. The corresponding nonterminal is then based on the central node instead. The fact that the top node of a subgraph is its point of contact with scopal operators is a fixed property of DMRS, guaranteeing the correct reconstruction of the full representation based on the generalised template.
To illustrate the issue, let us compare the following examples:
(96) Sam complained because there were no potatoes, but he made supper anyway (Ex- ample 95).
(97) Gollum cried and raged because Sam cooked the fish. (Fig. 5.14).
Both of these examples represent the interaction of a coordination and a subordinating conjunction. For the first one, both the chunk centre of the left coordinate and its top node
Fig. 5.13 DMRS for The rider s left Rohan, with Eowyn hiding amongst them. Fig. 5.14 DMRS of Gollum cried and ra g ed because Sam cook ed the fish.
5.4 Realization with templates 119
Fig. 5.15 DMRS for a fragment late for the bus.
Fig. 5.16 Functional graph for Exam-
ple 97. Fig. 5.17 Template for subordinating con-junction with because.
participate in the functional graph and the associated template (Fig. 5.9) because of the double coordination links. On the other hand, the functional graph of the second example uses only a single node from the main clause (Fig. 5.16). If we directly delexicalise the functional graph, we get a template in which the main clause chunk is represented by a coordinator node. This is, however, a missed generalization opportunity. Chunking Example 98 requires the same operations but would be represented by a different template:
(98) Gollum cried because Sam cooked the fish.
The left coordinates in both examples are finite clauses as determined by their central nodes (_cry_v_to). We can describe them by a single template (Fig. 5.17), which includes a nonterminal based on a delexicalised chunk centre, instead of the literal node participating in the represented link.
Although it is possible to work with fully delexicalised templates, our approach to realization requires a partially lexicalized version, because inter-chunk operators are only realized through surface templates based on training data (§5.4.4). Even though the two templates in Figures 5.17 and 5.18 could be merged by replacing their top operators with a nonterminal, e.g. _?_x_?, their corresponding surface templates have to be differentiated: A because B/Because B, A and A if B/If B A. We also retain the original predicate information for chunk centres which introduce other chunks as their scopal operators (Config. 3, §5.1.3). This allows us to distinguish nodes with compulsory scopal arguments, which we previously recognised based on their entries in the grammar lexicon (§3.2.3).
Within these constraints, we attempt to maximize the level of generalization. All tensed preposition, adverb/adjective and verb nodes are fully delexicalised, apart from an underspec-
Fig. 5.18 Template for subordinated clauses introduced by if.
ifiedtensed value as their tense property, because they are centres of standard finite clauses that are grammatically equivalent. Other tensed nodes also preserve their part-of-speech tag if they have real predicates, or their predicate name for grammar predicates. Untensed nodes retain all of their node properties during the nonterminal conversion, with the exception of underspecified lemma and sense for real predicates. We also fully delexicalise all nodes with instance variables (§2.3.1). Since they do not contribute to chunking decisions, they are stripped of all specific predicate and node properties, other than the type of intrinsic variable.
We accommodate the impact of modality operators on the grammatical properties of chunks by allowing nonterminals of central nodes to inherit node properties from their in-chunk ARG1 scopal predecessors (cf. §5.4.1).
Modifiers of inter-chunk operators are a recurring source of complication in semantic chunking (cf. §3.5). They can be of unrestricted forms and sizes, inflating the number of potential templates without contributing much information about the chunked construction. We limit their impact on templates by removing all but the topmost node of their subgraphs from the template. For example, if a coordination node is modified by a prepositional phrase, the corresponding template will only retain a delexicalised preposition node. We base this simplification on the assumption that similar modifiers interact with their arguments in the same way (cf. §5.4.4). Degree modifiers applied directly to operators, e.g. in only because, are not delexicalised and are an integral part of a template because of their contribution to surface templates (§5.4.4). They can be readily recognised because of the shape of their predicates (_?_x_deg).
5.4 Realization with templates 121 Construction Count _and_c (tensed) 114 _but_c (tensed) 40 implicit_conj (tensed) 36 _in+order+to_x 33 _and_c (untensed) 28 subord 27 _as_x 23 _when_x 20 _or_c 17 implicit_conj (untensed) 13 Table 5.1 Operators with the largest vari- ety of templates Construction Count subord 78 clausal_and_c 65 clausalimplicit_conj 52 _in+order+to_x 48
_while_x with a finite clause 34 be with a clausal complement 32
clausal_but_c 32
VP_and_c 31
_if_x 30
_although_x with a finite clause 30 Table 5.2 Templates with the most appli- cations