The Semantic Interface (SEM-I) is a specification of the ‘vocabulary’ and associated wellformedness constraints used in ERG meaning representations. In particular, the SEM-I inventories (a) the hierarchy of variable types; (b) the range of variable properties, associated value constraints, and appropriateness relative to variable types; and (c) the hierarchy of semantic predicates, each paired with its synopsis (‘argument frame’).
The notion of the SEM-I in the ERG continues a tradition of explicit interface specification (that can be interpreted independent of the core of the grammar) dating back to at least the Verbmobil project (in the closing decade of the past century). In DELPH-IN, this concept was further developed in the DeepThought and LOGON projects, among others, and some foundational reflections have been published by Flickinger et al. (2005). At about the same time, StephanOepen developed the software support (as part of the LKB environment) to largely automatically extract the semantic interface specification from the grammar sources. For the past ten or so years, each ERG release has provided an up-to-date SEM-I, but some sections were missing or incomplete until the ERG 1214 release (in mid-2016).
A High-Level Tour
As of the 1214 release, the ERG SEM-I is comprised of four files, viz. erg.smi, providing (a) and (b) from the above list, plus a few manually curated parts of (c); hierarchy.smi, setting up a partial (multi-rooted), multiple-inheritance hierarchy of semantic predicates; abstract.smi, listing the inventory of abstract predicates; and surface.smi, spelling out the (much larger) inventory of surface predicates. For the distinction between abstract vs. surface predicates, please see the ErgSemantics/Basics page.
Syntactically, the SEM-I implements a relatively lightweight syntax that aims to balance readability for both humans and machines.
variables: u. i < u. e < i : PERF bool, PROGR bool, MOOD bool, TENSE tense, SF sf.
properties: tense. tensed < tense. past < tensed. pres < tensed. fut < tensed. untensed < tense.
predicates: abstract_q : ARG0 x, BODY h, RSTR h. existential_q < abstract_q. universal_q < abstract_q.
Reflections on SEM-I Construction
When creating the files hierarchy.smi, abstract.smi, and surface.smi automatically from the grammar-internal type hierarchy, a set of heuristics is applied to decide which predicates to expose in the external interface. In the 1214 release, the set of abstract SEM-I predicates comprises some 120 entries, whereas there are some 500 abstract, non-glb types in the grammar-internal predicate hierarchy. In other words, the SEM-I is masking a large number of distinctions that the ERG makes internally (often for reasons of controlling syntactic combination and composition).
In a nutshell, there are three main reasons for including predicates in the SEM-I, viz. (a) observing a predicate in the semantics associated with a grammar entity (lexical items and rules); (b) specializing a predicate from (a) in the grammar-internal hierarchy; and (c) manual stipulation in either the top-level SEM-I file (erg.smi) or a small set of manual patches (maintained in 1214 as patches.lisp in the same directory). Quantitatively, the vast majority of predicates are associated with grammar entities, and of these, in turn, the by far largest part are surface predicates. Inclusion of type (b) predicates is motivated by the grammar at times specializing predicate (sub-)senses during parsing (i.e. in a process one might call co-composition); for example, verbs of motion can take an optional complement which has to be interpreted as a directional, e.g. She jumped in the lake in Berlin.
Note that (in the 1214 release of the ERG) we take advantage of the conceptual separation of the SEM-I predicate hierarchy vs. the grammar-internal type hierarchy, to not only (a) hide many of the grammar-internal predicate distinctions; but also to (b) introduce additional abstractions not present in the grammar; and to (c) add some parent–child links between predicates that are ‘missing’ in the grammar. An example of (b) is the sub-division of quantifiers into broad classes of existentials vs. universals (see above) and the (playful) addition of a predicate abstraction (called nn) over different ways of realizing an underspecified relation between two nominals, e.g. (generated from the sample input in nn.mrs):
A jungle lion arrived. A jungle's lion arrived. A lion of a jungle arrived.
An example of (c) above, finally, is making the temporal senses of prepositions like at, in, and on specializations of the ‘general’ predicates associated with these prepositions, e.g.
_in_p_temp < temp_loc_sp & _in_p.
Such additional parent links arguably violate an assumption made grammar-internally, viz. that the ‘general’ predicates for prepositions like _on_p exclude the temporal usage, i.e. correspond to spatio-locative (e.g. stative or directional) usages only. However, an empty sub-sense field in predicates (as is the case in plain _on_p) conventionally interpreted much like a wildcard (i.e. sub-sense underspecification), and hence it seems likely that providers of generator inputs might expect a predicate like _on_p to enable realization of a temporal relation, as for example in on Monday. Without modification of the grammar-internal hierarchy, this expectation can be satisfied by including the additional parent link in the SEM-I predicate hierarchy.
‘Upward’ Extensions of the Predicate Hierarchy
Software Support in DELPH-IN Processors
In June 2016, ACE provides full support for MRS comparison and manipulation in terms of the SEM-I hierarchies; this ‘modern’ mode is enabled by configuring a 2016-style SEM-I, e.g. (in the 1214 release of the ERG):
semantic-interface-2016 := "../etc/erg.smi".
At the same time, the LKB has migrated predicate processing (e.g. in MRS comparison, transfer, and indexing grammar entities by semantic contributions) to optionally be based on the SEM-I, whereas comparison of variable types and properties in pure SEM-I terms has yet to be implemented. This (semi-)‘modern’ mode in the LKB is closely tied to the recent rationalization of predicate serialization (see the Spring 2016 discussions on the ‘developers’ mailing list) and is thus activated as follows (e.g. in lkb/mrsglobals.lsp in the ERG):
(setf *normalize-predicates-p* t)