Millisecond Forums

Hi, I'm new to Inquest so please forgive my ignorance.
I'm trying to present stimuli in the following form 'a + b = ' and collect an open-ended response. I have specified two lists with the numbers 'a' and 'b' as follows:

<list firstnumbers>
/ items = (0,1,1, 1, 3, 3, etc.)
/ selectionmode = sequence
</list>

<list secondnumbers>
/ items = (1,1,1, 2, 2, 2, etc.)
/ selectionmode = sequence
</list>

Through an expression 'generatemathproblems', I generate the strings 'a + b = ' and this works fine. However, what I would like to do is not to select the numbers 'a' and 'b' in sequence, but randomly while preserving the correspondence between the numbers 'a' and 'b'. In other words, if say, the fourth item is selected for 'a', then I want 'b' to be the fourth item too; the fourth however should be randomly chosen. I tried to make a list 'stimulustracker' with a randomly chosen index between 1 and the maxnumberstimuli but list doesn't seem to support 'select':

<list stimulustracker>
/ poolsize = values.maxnumberstimuli
</list>

<list firstnumbers>
/ items = (0,1,1, 1, 3, 3, etc.)
/ select = list.stimulustracker.nextindex
</lis>

<list secondnumbers>
/ items = (1,1,1, 2, 2, 2, etc.)
/ select = list.stimulustracker.currentindex
</list>

Does anyone know how to solve this problem? Thanks a bunch!
--Marieke