 autom keyword classif inform retriev thi book monograph discuss main topic broaden surround context retriev textbook attempt justifi choic topic account concept provid elementari descript for instanc chapter 1 assum reader familiar idea keyword retriev consid relat kind retriev devic control thesauru descriptor set simpl class list document descript descript syntact structur exampl equal chapter 2 recal precis ratio characteris retriev perform justif argument unawar difficulti attent devot controversi rage round subject simpli point view main purpos reason ratio 