PauliChainExpand[exp] expands all Pauli chains with
explicit indices using linearity,
e.g. PCHN[CSIS[p1]+CSIS[p2]+m,i,j] becomes
PCHN[CSIS[p1],i,j]+PCHN[CSIS[p2],i,j]+m*PCHN[1,i,j].
Overview, PauliChain, PCHN, PauliIndex, PauliIndexDelta, DIDelta, PauliChainJoin, PauliChainCombine, PauliChainFactor.
PCHN[(CSIS[p] + m) . CSI[a], i, j]
PauliChainExpand[%]\left(\left(\overline{\sigma }\cdot \overline{p}+m\right).\overline{\sigma }^a\right){}_{ij}
m \left(\overline{\sigma }^a\right){}_{ij}+\left(\left(\overline{\sigma }\cdot \overline{p}\right).\overline{\sigma }^a\right){}_{ij}