$BreitMaison is a legacy switch for the
Breitenlohner-Maison-t’Hooft-Veltman scheme.
Use FCSetDiracGammaScheme to specify a scheme for
handling Dirac matrices in dimensional regularization and
FCGetDiracGammaScheme to check the current setting.
Overview, FCSetDiracGammaScheme, FCGetDiracGammaScheme.