[TODO: describe; rename the command to something more tclish?] Consequence of NRE.
Lars H: A thought: how is this different from what can be had through the age-old technique of creating a dummy local variable and putting an unset trace on it? Basic implementation:
proc atProcExit {script} {
upvar 1 atProcExistScripts arr
set arr($script) {}
trace add variable arr($script) unset "$script\n\#"
}