[Groff] figurate text in groff and TeX

[Doug McIlroy]

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