check if handler exist before using it

[Mark Wieder]

Richard-

> But I must admit that the circumstances in which such a mechanism was 
> useful have been very rare for me.   How often do you need to call a 
> handler without knowing whether it's present?

I actually end up doing this quite often: one behavior if a secondary stack 
is present, another behavior if it isn't. That allows me to add substacks as 
optional components and not throw errors if they aren't in a given 
distribution.

-- 
 Mark Wieder
 mwieder at ahsoftware.net