TODO.

nprimitives

int nprims = nprimitives(1);
for(int i = 0; i < nprims; ++i)
{
    int nvtx = primintrinsic(1, "vertexcount", i);
    vector pos = { 0, 0, 0 };
    if(nvtx > 0)
    {
        for(int j = 0; j < nvtx; ++j)
        {
            int ptidx = vertexpoint(1, vertexindex(1, i, j));
            pos += point(1, "P", ptidx);
        }
        pos /= nvtx;
    }
    int pt = addpoint(geoself(), pos);
}