"Josiah Carlson" jcarlson@uci.edu wrote in message news:20070421112051.63AB.JCARLSON@uci.edu...
Including a list of 'acceptable' classes as Terry has suggested would work, but would generally be superfluous.
I realized that later. The main use would be to improve the error message, or allow introspection ("Sdict, what can I put in you?").
tjr