+/** Calculate the trace of an expression containing gamma objects with
+ * a specified representation label. The computed trace is a linear
+ * functional that is equal to the usual trace only in D = 4 dimensions.
+ * In particular, the functional is non-cyclic in D != 4 dimensions when
+ * gamma5 is involved.
+ *
+ * @param rl Representation label */
+ex dirac_trace(const ex & e, unsigned char rl = 0);
+