Annotations are relative

Most systems that have been developed for annotation of data assume a two-level structure in which annotation is superimposed on, and separate from, the data. However there are many cases in which an annotation may itself be annotated. For example threads in e-mail and newsgroups allow the imposition of one comment on another; belief annotations can be compounded; and valid time, regarded as an annotation can be freely mixed with belief annotations (at time t₁, B₁ believed that at time t₂, B₂ believed that...).

In this paper we describe a hierarchical model of annotation in which there is no absolute distinction between annotation and data. First, we introduce a term model for annotations and, in order to express the fact that an annotation may apply to two or more data values with some shared structure, we provide a simple schema for annotation hierarchies. We then look at how queries can be applied to such hierarchies; in particular we ask the usual question of how annotations should propagate through queries. We take the view that the query together with schema describes a level in the hierarchy: everything below this level is treated as data to which the query should be applied; everything above it is annotation which should, according to certain rules, be propagated with the query. We also examine the representation of annotation hierarchies in conventional relational structures and describe a technique for annotating datalog programs.