Sure. This is all fine. But also straightforward.
>So, I was wondering if there is something inside FC...
Actually, Frederik added ExplicitLorentzIndex and some
rules for Eps.
>From the source code of Eps.m :
lor := lor = MakeContext["LorentzIndex"];
(*Added ExplicitLorentzIndex. 21/9-2002. F.Orellana*)
exlor := exlor = MakeContext["ExplicitLorentzIndex"];
mom := mom = MakeContext["Momentum"];
Eps[a__?(MatchQ[#,_Integer|exlor[_Integer]]&), ru___Rule] := Signature[{a}];
Eps[a___, n1_. (lor|exlor)[mu_,___], b___, n2_. (lor|exlor)[mu_,___],c___ ] := 0 /;
NumberQ[n1 n2];
Eps[a___, n1_. mom[mu_,___], b___, n2_. mom[mu_,___],c___ ] := 0 /;
NumberQ[n1 n2];
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