$Larin is a legacy switch for the Larin-Gorishny-Atkyampo-DelBurgo
scheme. The modern way is to use FCSetDiracGammaScheme to
specify a scheme for handling Dirac matrices in dimensional
regularization and FCGetDiracGammaScheme to check the
current setting.
Overview, FCSetDiracGammaScheme, FCGetDiracGammaScheme.