Representing Implicit Positive Meaning of Negated Statements in AMR (2021)

Abstract: Abstract Meaning Representation (AMR) has become popular for representing the meaning of natural language in graph structures. However, AMR does not represent scope information, posing a problem for its overall expressivity and specifically for drawing inferences from negated statements. This is the case with so-called “positive interpretations” of negated statements, in which implicit positive meaning is identified by inferring the opposite of the negation’s focus. In this work, we investigate how potential positive interpretations (PPIs) can be represented in AMR. We propose a logically motivated AMR structure for PPIs that makes the focus of negation explicit and sketch an initial proposal for a systematic methodology to generate this more expressive structure.

## introduction there has been a recent and growing interest in representing the meaning of natural language in graph structures. one semantic framework that has become popular is the semantic representation language of abstract meaning representation (amr) @xcite . amrs are attractive for their ease of readability, and they are intuitive representations due to their focus on predicate-argument structures. additionally, annotating meaning with graph structures is easier than following many traditional formal semantic frameworks @xcite and often allows for increased expressive capacity. an important application for meaning representations are inference tasks that play a crucial role in many nlp tasks such as information extraction and question answering @xcite . such inference tasks are where amr often falls short due to its lack of precise scope representation. for example, from (1-a) it can be inferred that "the boy stole the money" and hence the boy is a thief; from (1-b), it can be inferred that "the boy did not steal the money" and is likely not a thief. yet, the information provided by the scope of the negation alone is not sufficient when drawing inferences. negated statements can convey implicit positive meaning, or potential positive interpretation (ppi) @xcite , that is usually understood intuitively by humans. for example, an intuitive interpretation for (1-b) is that "someone other than the boy stole the money yesterday" (and the boy is not a thief). it could also be inferred, however, that "the boy stole something other than money," (and is still a thief) depending on whether the boy or the money is the focus of the negation. as a third alternative, if the focus of the negation is yesterday, we can indeed infer that "the boy stole the money," though we do not know when. both scope and focus of negation need to be detected to understand these nuanced interpretations and draw correct inferences, an exisitng weakness in many current natural language inference (nli) tasks @xcite . the goal of this paper is therefore (i) to investigate how the ppis of negative statements can be represented in amrs and (ii) to investigate whether these structures can be generated from the amr of the negated sentence in a systematic way based on the work by @xcite and @xcite . to this end, we propose a novel, logically-motivated amr structure that makes both scope and focus of negation explict. we additionally sketch an initial proposal for how to transform existing amrs into this more expressive format. initial assessment results support previously made observations that amr lacks expressive capacity when it comes to scope. this lack of expressivity is especially problematic for ppis whose concrete meaning results from an interaction of the negation and information structure. our methodology semantic role dependency potential positive interpretations 1 arg 0 nsubj [someone] stole my money yesterday, but not the young boy 2 arg 1 obj the young boy stole [something] yesterday, but not my money 3 verb root the young boy [some verb] my money yesterday, but not stole 4 argm -tmp obl:tmod the young boy stole my money [at some point in time], but not yesterday 5 nsubj the young [some person] stole my money yesterday, but not the young boy 6 amod the [some adjective] boy stole my money yesterday, but not the young boy 7 nmod:poss the young boy stole [someone's] money yesterday, but not my money 8 obj they young boy stole my [something] yesterday, but not my money positive counterpart: the young boy stole my money yesterday. this paper is structured as follows. section 2 presents background about focus of negation, previous work on generating ppis and about scope in amr. based on this, we propose an approach to represent the ppis of a negated statement in amrs and a methodology to systematically generate these amrs (section 3) which we then verify with corpus examples (section 4). in section 5 we conclude and discuss next steps. ## method the goal of this exploratory work is to investigate (i) how the ppis of a negated statement can be represented in amrs and (ii) if it is possible to generate them systematically from the original amr of the negated statement for increased expressivity. we employ a combination of the semantic role based @xcite and the syntactic dependency based approach @xcite as the starting point for this investigation. for simplicity, we ignore tense as well as semantic differences between definite and indefinite determiners. ## findings in the following section, the validity of the initial proposal for the representation of ppis of negated statements in amrs and for the methodology to generate them will be verified by means of examples and extended and adapted if necessary. for reasons of space, only some of the ppis are presented for the example statements. (3) ## conclusion & future work the way in which negation is represented in amrs does not allow for accurate interpretation of scope or the ability to disambiguate between the potential foci of the negation. the goal of the current work was therefore, to investigate how the ppis of negated statements can be represented in amrs in order to explicitly represent the focus of negation and whether these representations can be generated in a systematical way. we proposed a logically motivated representation for the ppis where the negation is attached directly to the root node of the focus. as future work, we plan to investigate our proposed approach on sentences from existing amr corpora. working through example negated statements has shown that representing positive meaning adequately in amrs is a complex task especially when quantifier and scope interactions are involved. the results support the need for an extension of amr that is able to handle quantifiers and scope phenomena in general, and negation in particular, in a systematic and explicit way that is suitable for inference tasks. representing focus explicitly in amrs does not solve the problem that modelling negation simply as a relation is inappropriate for drawing inferences. as a next step, then, we hope to combine our proposed approach with an existing approach that extends amr's scope capacities, specifically by investigating the use of @xcite 's approach for determining which nodes to consider as potential foci and by translating the contrast-01 concept and the existential placeholder subgraphs into fol.