5. A method executed by one or more computers for identifying free-text documents, the method comprising: maintaining a knowledge source storing 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; generating under control of the one or more computers, indexing concepts for the free-text documents stored in a database, the indexing concepts being generated based on one or more words from the predefined vocabulary set, wherein the generating of the indexing concepts includes: maintaining a plurality of data structures mapping each of the plurality of concepts in the knowledge source to all words appearing in the concept; receiving a particular one of the free-text documents; 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 returning the one or more identified concepts as candidate index concepts for the particular one of the free-text documents; receiving, under control of the one or more computers, an input query including an original query concept and at least one scenario identifier; automatically generating, under control of the one or more computers, an expanded input query including both the original query concept and one or more additional query concepts, the one or more additional query concepts being selected from one or more first particular semantic groupings of the concepts in the knowledge source that have a specific relationship link with a second particular semantic grouping of the concepts containing the original query concept, the specific relationship link being identified by the at least one scenario identifier, wherein the automatically generating of the expanded input query further includes: filtering candidate expansion concepts not included in the one or more first particular semantic groupings of the concepts in the knowledge source; and returning the one or more additional query concepts based on the filtering; accessing the database of free-text documents; comparing the expanded input query with the indexing concepts for the free-text documents; and returning one or more of the free-text documents that satisfy the expanded input query based on the comparison.
6. The method of claim 5 wherein a sequence of appearance of the words in the particular one of the free-text documents is a free-permutation of the words in the set.