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 slightly different that the notation
of PV, i.e. the naming convention is more
systematic. The formula implemented
can be found in,

@article{Ellis:2011cr,
      author         = "Ellis, R. Keith and Kunszt, Zoltan and Melnikov, Kirill
                        and Zanderighi, Giulia",
      title          = "{One-loop calculations in quantum field theory: from
                        Feynman diagrams to unitarity cuts}",
      journal        = "Phys. Rept.",
      volume         = "518",
      year           = "2012",
      pages          = "141-250",
      doi            = "10.1016/j.physrep.2012.01.008",
      eprint         = "1105.4319",
      archivePrefix  = "arXiv",
      primaryClass   = "hep-ph",
      reportNumber   = "FERMILAB-PUB-11-195-T",
      SLACcitation   = "%%CITATION = ARXIV:1105.4319;%%"
}

In addition in this version, as in the original PV paper, 
the tensor integrals are expanded in the external momenta, p_i