ExplicitPauliIndex[ind]
is an explicit Pauli index,
i.e., ind
is an integer.
Overview, PauliChain, PCHN, PauliIndex, PauliIndexDelta, PIDelta, PauliChainJoin, PauliChainCombine, PauliChainExpand, PauliChainFactor.
[SI[i], 1, 2] PCHN
\left(\bar{\sigma }^i\right){}_{12}
[SI[i], 1, 2] // FCI // StandardForm
PCHN
(*PauliChain[PauliSigma[LorentzIndex[i]], ExplicitPauliIndex[1], ExplicitPauliIndex[2]]*)