Introduction

Abstract

Part-whole hierarchies are a natural way for humans to organize and represent entities in order to manage and reason about the world. Often these hierarchies can be seen in terms of objects composed of other objects. For instance, a document may be seen as a composite object made up of parts. Some of these parts may be sections which themselves are composite objects containing paragraphs. The notion of composite objects is one which is gaining strength in a number of areas. For instance, in the object modeling domain whole-part structures of objects are recognized as one of the important organization techniques and in most object-oriented modeling approaches part-whole relations can be represented in a specialized way (e.g. [CY91, RBPEL91]). In the object-oriented databases area there have been efforts to represent and manage composite objects. Several object-oriented database management systems support some kind of part-whole and some of them allow for variations of the properties of part-whole. Support for composite objects is even regarded as a necessary facility for more complex database systems such as engineering and office databases [St*90, At*89, Cat91]. In document management systems there are international standards such as SGML and ODA requiring documents to be defined in terms of a structure of parts.