This article contains a presentation and overview of the papers that I have submitted for the degree doctor scientiarum (dr. scient.).

The introduction relates the subject matter of the submitted papers to current discussions in computer science. Section two gives a brief account of the research area in question, how it has developed over the last 25 year~, and its current status.

Section three presents and discusses the results structured according to the frame of reference given in section two. For each sub-area the central issues are introduced. Then the results obtained are presented; the practical as well as the theoretical. Finally, a short comparison and evaluation in relation to relevant literature is made.

Section four contains a short discussion of the methods used, and section five presents ideas for future research.