xlRateHelperSelection() question

[gj!!!]

hi all,

quantlib rules! but now my question:

xlRateHelperSelection() selects, given certain ratehelpers and criteria, the ratehelpers which can be used in qlPiecewiseYieldCurve()

can someone tell me how the input-instruments are actually determined in the function xlRateHelperSelection()

what kind of methodology is used here?  

i checked the cpp source but i could not find a trace of this function...

any help greatlly appreciated.

thanks! brgds GJ