ToDiracSigma[exp, x, y] substitutes the neighboring Dirac matrices x and y by DiracSigma and the metric tensor.
Overview, DiracGamma, DiracSigma, DiracSigmaExplicit.
GA[\[Mu], \[Nu]]
ToDiracSigma[%, GA[\[Mu]], GA[\[Nu]]]\bar{\gamma }^{\mu }.\bar{\gamma }^{\nu }
\bar{g}^{\mu \nu }-i \sigma ^{\mu \nu }
GA[\[Mu], \[Nu], \[Alpha], \[Beta], \[Rho], \[Sigma]]
ToDiracSigma[%, GA[\[Alpha]], GA[\[Beta]]]\bar{\gamma }^{\mu }.\bar{\gamma }^{\nu }.\bar{\gamma }^{\alpha }.\bar{\gamma }^{\beta }.\bar{\gamma }^{\rho }.\bar{\gamma }^{\sigma }
\bar{g}^{\alpha \beta } \bar{\gamma }^{\mu }.\bar{\gamma }^{\nu }.\bar{\gamma }^{\rho }.\bar{\gamma }^{\sigma }-i \bar{\gamma }^{\mu }.\bar{\gamma }^{\nu }.\sigma ^{\alpha \beta }.\bar{\gamma }^{\rho }.\bar{\gamma }^{\sigma }