16. A method of operating a computerized information retrieval system where information is retrieved from a database containing documents in response to user queries, the method comprising: processing a natural language query specifying information to be retrieved to generate an alternative representation of the query, the alternative representation including generating a logical representation incorporating terms and phrases found in the query and assigning weighted Boolean scores to the query terms; matching the alternative representation of the query, against the database; for at least some documents in the database, generating a measure of relevance of the document to the query using a common alternative representation of the document and the alternative representation of the query; and displaying a list of documents whose measures of relevance are determined by the system to be sufficiently high; receiving user input specifying selection of at least a portion of at least some of the list of documents; using the alternative representations of the documents, or portions of documents, so selected, to generate a new query representation; and matching the new query representation against the database.
16. A method of operating a computerized information retrieval system where information is retrieved from a database containing documents in response to user queries, the method comprising: processing a natural language query specifying information to be retrieved to generate an alternative representation of the query, the alternative representation including generating a logical representation incorporating terms and phrases found in the query and assigning weighted Boolean scores to the query terms; matching the alternative representation of the query, against the database; for at least some documents in the database, generating a measure of relevance of the document to the query using a common alternative representation of the document and the alternative representation of the query; and displaying a list of documents whose measures of relevance are determined by the system to be sufficiently high; receiving user input specifying selection of at least a portion of at least some of the list of documents; using the alternative representations of the documents, or portions of documents, so selected, to generate a new query representation; and matching the new query representation against the database. 19. The method of claim 16 wherein: the alternative representation of the query includes a conceptual level defined by subset of a finite set of subject codes, the subset corresponding to subject codes interpreted by the system to characterize the subject contents of the query; and the user input specifies increasing or decreasing the subset of subject codes.