[Zope] Can't locate PyObject_SetItem

I am trying to track down the cause of a performance problem in ZCTextIndex (OkapiIndex is the leading contender - thanks to Dieter to for the tip!) and have gotten as far as the 'score' routine in okascore.c - which references a routine called 'PyObject_SetItem', for which I cannot locate the source routine. Does anyone know where the source for the 'PyObject_xxx' routines are stored (linux installation)? Thanks in advance, Jonathan