//* Compute the trace of 14 Dirac gamma matrices.
expr F = g(m1,m2,m3,m4,m5,m6,m7,m8,m9,m10,m11,m12,m13,m14);
{
  id g(?a) = g1()*g2(?a);
  repeat;
    repeat id g1(?a)*g2(m1?,m2?,?b)
      = d_(m1?,m2?)*g1()*g2(?a,?b) - g1(?a,m2?)*g2(m1?,?b);
    id g2(m1?) = 0;
  endrepeat;
  id g1()*g2() = 4;
}