- dirac_trace() handles arbitrary expressions (even unexpanded), including

- dirac_trace() handles arbitrary expressions (even unexpanded), including
  gamma5
- implemented color_trace()
- eps0123() creates a 4-dimensional epsilon tensor without checking the
  indices (so they may be D-dimensional); this may become a different class
  soon, but for now all the implemented properties of the standard epsilon
  tensor also apply to eps0123