When i added documentation blocks to these callbacks, the call back def code
started complaining that the body {\n} is not empty. Replaced this with the
"-" empty body marker. Interestingly:
ad_proc -callback {
params
} {
}
apparenly does not trigger the "non-empty body" error. Apparently the callback def
only checks if there's a docblock passsed to ad_proc ... a "gotcha".