This paper is a major revision combining and extending two other papers [1,2].

This paper has mainly been written when the author was a visiting scientist at Xerox Palo Alto Research Center.

Actually, its upper bound for vertical space is set to a height which would allow to display the whole list of direct methods at once provided that sufficient screen space is available.

Layout forms are usually defined as macros evaluating the right expressions (in the right scope).

This is an indication that the available space is not sufficient to fulfill the space requirements. Items could extend beyond the boundary of their surrounding box.

The capacity of a jug be temporarily unlimited.

This is needed for frame-boxes, which may otherwise not exactly fit to the lower border of their box.

Volker Haarslev
Tue Jun 11 13:34:48 MET DST 1996