Onto.PT: Towards the Automatic Construction of a Lexical Ontology ...

4.2. A large lexical network for Portuguese 63
that, in DLP, frequently denote the relations we wanted to extract 13 . The grammars
were created manually, after the analysis of the structure and vocabulary of
the DLP definitions, and the identification of regularities.
In order to reproduce the grammar creation procedure for extracting relations
from the other dictionaries, we also analysed the structure of their definitions. This
analysis showed that most of the regularities used in the DLP definitions were preserved
in DA and Wiktionary.PT, which meant that the grammars of PAPEL could
be reused with minor changes. Table 4.1 shows the frequency and the semantic
relation usually denoted by the most productive n-grams in the three dictionaries,
which are those frequent and suitable for exploitation in the automatic extraction of
semantic relations. In the referred table, some patterns extract the direct relation
(e.g. part-of) and others the inverse relation (e.g. has-part) but, during the extraction
procedure, all relations are normalised into the type agreed as the direct (e.g.
keyboard has-part key is changed to key part-of keyboard).
The few changes we made to the original grammars of PAPEL, include:
• The pattern o mesmo que was used in the extraction of synonymy relations.
• The keywords natural and habitante could change their order in the extraction
of place-of relations.
• Brazilian Portuguese specific orthography was considered in some patterns, as
they occurred in Wiktionary.PT. Words such as g^enero and ato were used,
respectively, for the extraction of hypernymy and causation relations.
In addition to the static patterns in table 4.1, two other productive rules were
included in the grammars for extracting relations from the three dictionaries:
• Synonymy can be extracted from definitions consisting of only one word or a
enumeration of words. See the following example:
talhar verbo gravar, cinzelar ou esculpir
→ gravar synonym-of talhar
→ cinzelar synonym-of talhar
→ esculpir synonym-of talhar
• As most noun definitions are structured on a genus and differentia (see section
3.2.1), we identify the genus as a hypernym of the definiendum, which
might eventually be modified by an adjective. The following are examples of
the application of this rule:
island^es nome língua germ^anica falada na Isl^andia
→ língua hypernym-of islandês
pantera nome grande felino de o g^enero Panthera
→ felino hypernym-of pantera
The second rule does not apply, however, when the definition starts by a so
called “empty head” (Chodorow et al., 1985; Guthrie et al., 1990), which is usually
exploited in the extraction of other relations. The list of considered “empty
heads” includes words such as acto, efeito (used for the extraction of the causation
relation), qualidade (has-quality), estado (has-state), parte (part-of),
13 The grammars of PAPEL are freely available from http://www.linguateca.pt/PAPEL/
(September 2012)