This is a tensor reduction routine
based on the work of Passarino and Veltman

@article{Passarino:1978jh,
      author         = "Passarino, G. and Veltman, M.J.G.",
      title          = "{One Loop Corrections for e+ e- Annihilation Into mu+ mu-
                        in the Weinberg Model}",
      journal        = "Nucl.Phys.",
      volume         = "B160",
      pages          = "151",
      doi            = "10.1016/0550-3213(79)90234-7",
      year           = "1979",
      reportNumber   = "Print-79-0284 (UTRECHT)",
      SLACcitation   = "%%CITATION = NUPHA,B160,151;%%",
}

The notation for the form factors 
is however different that the notation
of PV. In particular the tensor integrals are functions
of the off-set momenta,q_i, not the external momenta, p_i.