In many existing hypertext systems, a navigation mechanism is provided as the main facility so that readers search the contents. In order to expand the ability to search, in this paper we propose a query mechanism to be used in addition to the ordinary navigation mechanism.We can express advanced abilities of search by combining the query mechanism with the navigation mechanism. These query expressions can be used to provide the user's point of view besides the designer's point of view, which was previously defined by the links among nodes. Especially,an environment that changes dynamically in correspondencewith the user's navigation can be described when one uses our query mechanism. Since the mechanisms proposed in this paper are defined in a formal way, they can be applied to various practical hypertext systems.
