It would be great if there was a definitive guide to writing simplifying functions.
There are plenty of examples, but no how-to manual that I know of; some topics
that come to mind:
(1) nouns and verbs,
(2) autoloading simplifying functions,
(3) simplification--how much is too much,
(4) reflection rules,
(5) binary64 & bigfloat evaluation,
(6) extras: gradef, conjugates, limits, TeX properties, antiderivatives,
(7) declaring function properties (complex, ...),
(8) domain and wrong number of argument errors,
--Barton