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We distinguish between these two type of relations as
persistent
or
contextual
.Per-
sistent relations are those that form part of the lexical knowledge which are valid within
the context of a particular discourse as well as all other discourses. On the other hand
contextual relations are transient, and may be valid only for the duration of a single dis-
course, for example, “a cup on a table” or “John has a knife”. The contextual relations
are expressed as either a verb or a preposition, relating two entities in the discourse
being processed. In order to resolve all bridging anaphora in a discourse, we need to
identify both persistent as well as the contextual relations. The persistent relations can
be expressed either explicitly or assumed as part of the lexicon. On the other hand,
the contextual relations have to be expressed explicitly via verbs and prepositions. The
question now is which verbs and prepositions represent the anaphoric relations.
As argued earlier, the semantic relations used by bridging anaphora are the same as
those used for compound noun generation, hence for this study we adopted the set pro-
posed in [18]. The set of relations consist of CAUSE, HAVE, MAKE, USE, BE, IN,
FOR, FROM and ABOUT. In order to define a complete framework for anaphora in-
terpretation, we needed to do two modifications to the nine relations from [18]. Both of
these modifications were done in order to be able to better interpret and represent plural
anaphoric nouns. This was done by introducing a new relation named ACTION, and
by splitting the existing BE relation into BE-INST and BE-OCCR. These are explained
next.
When two or more entities in a discourse are participating in the same or similar
event, they can be referred to as a unit by a collective NP in the context of the discourse.
The entities in the same or similar action can be expressed by the conjunction
and
or
described by two different clauses. For instance in the sentence “The coastguard and
Lion Foundation Rescue helicopter were called out.”, the entities
coastguard
and
Lion
Foundation Rescue helicopter
are related to each other by the virtue of participating in
the same action. Similarly, the clauses “the truck rolled down the hill” and “the ball
rolled down the hill” would enforce the same relation between
truck
and
ball
since they
are both engaged in the same action (roll). This relation between truck and ball is only
valid for the context of the discourse, hence this relation is contextual. We describe this
contextual relation as the ACTION relation which relates entities participating in events
which are identified to be same or similar. The ACTION is used to describe an NP such
as
runners
used to refer to
fox
and
Peter
from the context clause “The fox and Peter
were running”.
The second modification involved defining a finer grained BE relation in order to
interpret existence of plurals in a different form. We split Levi's BE relation into BE-
OCCR and BE-INST to distinguish between direct co-reference or identity relation
and an instance relation. In a BE-OCCR relation an NP directly forms a one-to-one
co-reference to another NP, eg.
John/he
and
John/the driver
. The BE-INST relation
represents cases where an anaphor refers to a plural antecedent, in a partial capacity,
for example,
both trucks/northbound truck
. In this case the NP
northbound truck
is an
instance of
both trucks
which is distinct from a co-reference relation. It can be argued
that all subset/ superset relations such as
John/driver
(John is an instance of driver) and
car/vehicle
(car is and instance of vehicle) is an instance relation. However we consider
these as BE-OCCR relation since they
function
to identify the entity. Hence in the
