9. In a document retrieval system storing more than one library document, an apparatus for retrieving library documents matching the content of a sequence of query words, comprising: (a) means for storing a set of equivalent words for each of the query words, each of said equivalent words being stored with a corresponding word equivalence value; (b) means coupled to said storage means for computing a relevance factor for a library document, comprising: (i) first means for receiving a set of matching criteria; (ii) second means coupled to said first means for locating target sequences of words in a library document that match the sequence of query words, and equivalence thereof, according to said matching criteria; (iii) third means coupled to said second means for evaluating similarity values of said target sequences of words, each similarity value being evaluated as a function of the equivalence values of words included in the corresponding target sequence; and (iv) fourth means receiving said similarity values for computing said relevance factor.
13. The apparatus as in claim 9 wherein said matching criteria include the span of a sequence of words in the library document that matches the sequence of query words.