Question: When does OpenMaple call queryInterrupt?

(This question applies to the Win32 version of Maple 15.)

I've got a python wrapper for the OpenMaple interface, and I've managed to get most of the callback functions in the MCallBackVector structure passed in to StartMaple to work, but I've never (ever) seen the queryInterrupt callback called.

To investigate this, I modified the 'omexample.c' example (found in the <Maple install Dir>\samples\OpenMaple\cmaple directory installed with the product), changing the 'queryInterrupt' function to print a short message when called.  I then built the application and ran it, passing in the statement

for idx from 1 to 1000000000 do 1/idx end do;

Again, I'm not seeing queryInterrupt called by OpenMaple.

Does anyone know when this callback is actually used (is it only for certain Maple statements, for example)?  Do I need to set certain parameters in Maple for this to work?  I really would like to be able to safely interrupt long-running calculations (or calculations where Maple begins consuming large amounts of RAM), but since the queryInterrupt callback isn't being called I don't see how I can do so...