Relations

In order to form a natural language sentence or a UNL graph, nodes are inter-related by relations. In the UNL framework, there can be three different types of relations between nodes:

• the linear relation L, which defines the order of the elements in a list
• syntactic relations (such as adjunct of the noun phrase, complement of the verbal phrase, specifier of the adjective phrase, etc.)
• semantic relations (such as agent, object, manner, instrument, etc.)

Basic Symbols

Basic symbols used in the UNL framework

Symbol	Definition	Example
( )	node	(%a)
" "	string	"went"
[ ]	natural language entry (headword)	[go]
[[ ]]	UW	[[to go(icl>to move)]]
//	regular expression	/a{2,3}/ = aa,aaa
rel(x;y)	relation	agt(kill;Peter)
^	not	^a = not a
{ | }	or	{a|b} = a or b
%	index for nodes, attributes and values	%x
:	scope ID	:01
#	index for sub-NLWs	#01
=	attribute-value assignment	POS=NOU
!	rule trigger	!PLR
&	merge operator	%x&%y
?	dictionary lookup operator	?[a]

Basic Concepts

Node

A node is the most elementary unit in the graph. It is the result of the tokenization process, and corresponds to the notion of "lexical item". At the surface level, a natural language sentence is considered a list of nodes, and a UNL graph a set of relations between nodes.

Relation

In order to form a natural language sentence or a UNL graph, nodes are inter-related by relations. In the UNL framework, there are three different types of relations: the linear (list) relation, syntactic relations and semantic relations.

Hyper-Node

A hyper-node is a sub-graph, i.e., a scope: a node containing relations between nodes.

Hyper-Relation

A hyper-relation is a relation between relations.

Notation

Relations are represented by the general syntax

rel:scope(arg1;arg2;...;argn)

Where

• rel is the name of the relation;
• scope is the scope of the relation;
• arg1, arg2, ..., are the arguments of the relation, i.e., nodes.

The main scope is 00 and it is not shown, by default:

• rel(arg1;arg2) is the same as rel:00(arg1;arg2) (i.e., the relation agt belongs to the main scope, i.e., the main graph)
• rel:01(arg1;arg2) (the relation rel belongs to the scope :01, i.e., a sub-graph (hyper-node) inside the main graph)

Types

In the UNL framework, there can be three different types of relations:

• the linear relation L expresses the surface (list) structure of natural language sentences
• syntactic relations express the syntactic (tree) structure of natural language sentences
• semantic relations express the semantic (graph) structure of UNL graphs

Examples

Examples of relations:

• ("a")("b") (a linear relation between two nodes: one having the string "a" and the other having the string "b"
• L("a";"b") (the same as above)
• VC(V;NP) (a syntactic relation VC between two nodes: one having the feature V and the other having the feature NP
• VC("a",V;"b",[[b]],LEX=N,NP) (a syntactic relation VC between two nodes: one having the string "a" and the feature V; and the other having the string "b", the UW b and the features LEX=N and NP)
• agt("kill";N) (a semantic relation between two nodes: one having the string "kill" and the other having the feature N.

Properties

The linear relation is always binary and is represented in two possible formats

• L(%x;%y) or
• (%x)(%y)

where L is the invariant name of the linear relation, and %x and %y are nodes.

Syntactic relations are not predefined, although we have been using a set of binary relations based on the X-bar theory.

Semantic relations constitute a predefined and closed set that can be found here.

Arguments of relations are not commutative.

The order of the elements in a relation affects the result:

(%x)(%y) is different from (%y)(%x)

relation(%x;%y) is different from relation(%y;%x)

Linear and semantic relations are always binary; syntactic relations may be n-ary

L(%x;%y) - linear relation

agt(%x;%y) - semantic relation

VH(%x) - unary syntactic relation

VC(%x;%y) - binary syntactic relation

XX(%x;%y;%z) - possible ternary syntactic relation

Inside each relation, nodes are isolated by semicolon (;).

VC(%x;%y)

VC(%x,%y)

Inside each relation, nodes may be referenced by any of its elements, isolated by comma (,)

("a")([b]) - linear relation between a node where string = "a" and another node where headword = [b]

L([[c]];D) - linear relation between a node where UW = [[c]] and another node having the feature D

VC(%a;%b) - syntactic relation between a node where index = %a and another node where index = %b

agt("a",[a],[[a]],A;"b",[b],[[b]],B) - semantic relation between a node having the feature A where string = "a" AND headword "a" AND UW = [[a]] AND another node having the feature B where string = "b" AND headword = [b] AND UW = [[b]]

Relations may be conjoined through juxtaposition

("a")("b")("c") - two linear relations: one between ("a") and ("b") AND other between ("b") and ("c")

agt(%x;%y)obj(%x;%z) - two semantic relations: one between (%x) and (%y) AND other between (%x) and (%z)

VC([a];[b]),VC([a];[c]) - conjoined relations must not be isolated by comma

Relations may be disjoined through {braces}

{("a")|("b")}("c") - either ("a")("c") or ("b")("c")

{agt(%x;%y)|exp(%x;%y)}obj(%x;%z) - either agt(%x;%y)obj(%x;%z) or exp(%x;%y)obj(%x;%z)

Syntactic and semantic relations may be replaced by regular expressions

/.{2,3}/(%x;%y) - any relation made of two or three characters between %x and %y

Differently from nodes, relations do not have elements (strings, headwords, features and indexes)

In rel("a",[a],[[a]],A;"b",[b],[[b]],B), the elements "a", "b", [a], [b], [[a]], [[b]], A and B belong to the arguments of the relation and not to the relation itself.