FCPauliIsolate[exp] wraps chains of Pauli matrices into
heads specified by the user.
FCPauliIsolate[y SI[i] + x PauliXi[-I] . SIS[p1] . PauliEta[I] . PauliEta[-I] . SIS[p2] . PauliXi[I], Head -> pChain]y \;\text{pChain}\left(\bar{\sigma }^i\right)+x \;\text{pChain}\left(\xi ^{\dagger }.\left(\bar{\sigma }\cdot \overline{\text{p1}}\right).\eta \right) \;\text{pChain}\left(\eta ^{\dagger }.\left(\bar{\sigma }\cdot \overline{\text{p2}}\right).\xi \right)