that one works as well. Thanks!
(Sorry for not answering earlier - I didn't get any notification from your reply, and in the meantime was busy.)
I have another strange behaviour to report, with the xPert package.
Defining a metric g and perturbation h, ToCanonical[ h[LI[1], -a, -b] Perturbation[ g[c,b] ] ] returns 0.
It seems that this is due to ToCanonical reordering the indices to h[LI[1], -a, b] Perturbation[ g[c,-b] ], and then Perturbation[ g[c,-b] ] = Perturbation[ delta[c,-b] ] = 0.
I haven't come up with a fix for this one, though, and for the moment just make sure to ExpandPerturbation[] everywhere.
Best, Markus