If two keyphrases are part of a combined keyphrase, the
structural information of that combined keyphrase can be
looked up in the ontology to figure out which component is
the main keyphrase. If such information is not available,
we assume that the keyphase goes first in writing order is
grammatically more important because it tell us more
information about the second keyphrase. When two
keyphrases are not part of a combined keyphrase, we can
estimate their importance based on their position in the
hierarchical relations tree. The keyphrase that goes deeper
in the tree would be more important since it has more
specific meaning and can help us process the query with
more precise.