Suppose one wants to control the shape of filled text so it fits in a figure, for example a triangle or disc. In groff one would set a trap to change indent and/or line length on every line. The traps could be set all at once in advance, or more elegantly by a self-resetting trap macro.
Examples of figurate text appear in Knuth's paper on the placement of line breaks. How would the changing indent and length parameters be expressed in TeX? (The question is motivated from thinking about how to do Knuth-ish line breaking in the extremely dynamic groff environment. Figurate text, especially when done by a self-resetting macro, is an example of path-dependent state variables. In general it's possible for the text itself to depend on path, but that's a challenge for another day.) Doug
