14. A system for identifying free-text documents, the system comprising: a knowledge source storing in a knowledge database a plurality of concepts formed based on one or more words in a predefined vocabulary set, the knowledge source maintaining a plurality of relationship links wherein each relationship link defines a relationship between a first semantic grouping of the concepts and a second semantic grouping of the concepts; a database of free-text documents; means for generating the indexing concepts for the free-text documents in the database, the indexing concepts being generated based on one or more words from the predefined vocabulary set wherein the means for generating the indexing concepts includes: a plurality of data structures mapping each of the plurality of concepts in the knowledge source to all words appearing in the concept; means for receiving a particular one of the free-text documents; means for identifying based on the plurality of data structures, one or more of the plurality of concepts in the knowledge source mapped to a set of words appearing in the particular one of the free-text documents; and means for returning the one or more identified concepts as candidate index concepts for the particular one of the free-text documents; means for receiving an input query including an original query concept and at least one scenario identifier; means for generating an expanded input query including both the original query concept and one or more additional query concepts, wherein the means for generating includes: means for identifying a specific relationship link in the knowledge source based on the at least one scenario identifier; means for identifying a first semantic grouping of the concepts containing the original query concept in the knowledge source; means for identifying a second semantic grouping of the concepts in the knowledge source associated with the first semantic grouping of the concepts via the specific relationship link; means for filtering candidate expansion concepts not included in the second semantic grouping of the concepts; and means for selecting at least one of the one or more additional query concepts from the second semantic grouping of the concepts based on the filtering; means for accessing the database of free-text documents; means for comparing the expanded input query with the indexing concepts for the free-text documents; and means for returning one or more of the free-text documents that satisfy the expanded input query based on the comparison.
14. A system for identifying free-text documents, the system comprising: a knowledge source storing in a knowledge database a plurality of concepts formed based on one or more words in a predefined vocabulary set, the knowledge source maintaining a plurality of relationship links wherein each relationship link defines a relationship between a first semantic grouping of the concepts and a second semantic grouping of the concepts; a database of free-text documents; means for generating the indexing concepts for the free-text documents in the database, the indexing concepts being generated based on one or more words from the predefined vocabulary set wherein the means for generating the indexing concepts includes: a plurality of data structures mapping each of the plurality of concepts in the knowledge source to all words appearing in the concept; means for receiving a particular one of the free-text documents; means for identifying based on the plurality of data structures, one or more of the plurality of concepts in the knowledge source mapped to a set of words appearing in the particular one of the free-text documents; and means for returning the one or more identified concepts as candidate index concepts for the particular one of the free-text documents; means for receiving an input query including an original query concept and at least one scenario identifier; means for generating an expanded input query including both the original query concept and one or more additional query concepts, wherein the means for generating includes: means for identifying a specific relationship link in the knowledge source based on the at least one scenario identifier; means for identifying a first semantic grouping of the concepts containing the original query concept in the knowledge source; means for identifying a second semantic grouping of the concepts in the knowledge source associated with the first semantic grouping of the concepts via the specific relationship link; means for filtering candidate expansion concepts not included in the second semantic grouping of the concepts; and means for selecting at least one of the one or more additional query concepts from the second semantic grouping of the concepts based on the filtering; means for accessing the database of free-text documents; means for comparing the expanded input query with the indexing concepts for the free-text documents; and means for returning one or more of the free-text documents that satisfy the expanded input query based on the comparison. 15. The system of claim 14 , wherein each of the semantic grouping of the concepts is identified by a semantic type.