2-5


          cate~ories or ooncept codes.  Thus a set 0£ semantically associated

          natural lang~~e terms comprised 0£ synonyms, £or example, can be

          mapped into a single element in the index langu~e; or a single

          natural langu~e term which has several connotations can be identified

          with a set 0£ elements in~the index langu~e (homonyms mi~ht be

          treated in this manner).  Figure 2.1 provides an illustration by means

          0£ an excerpt £rom the S~ART system thesaurus.

                 The notion 0£ a semantically based transformation on a set 0£

          reco~nizable (by machine) linguistic £eatures (word or stem types,

          phrases, etc.) can be generalized to include a variety 0£ the
                                                   13
          associations which such elements possess.   The index transformation

          may be described in this case by considering a multi-stage mapping.

          The £irst step consists in mapping the document into the set 0£ basic

          elements which describe it, e.g. into the set 0£ word types it contains.

          The second step is a transformation £rom these elements into a space

          0£ synonymous term groups i.e. into thesaurus categories. (The

          thesaurus mapping described above consists in applying these two basic

          transformations.)  Additional transformation stages may also be de£ined.

          Thus generic (inclusion). relations exist among semantic elements and

          these may be used to de£ine a set 0£ hierarchies.  A number 0£

          transformation can be de£ined based on a set 0£ such relations; thus

          a term which includes or. which is included by a given term may be~

          added to or may replace the related term in the document image.   The

          index image 0£ a document, there£ore, can~be modified to contain terms

          which are generically related to those detected, but not explicitly

          present in the input text.  Relation~s among index terms other than


)