where τ are Pauli operators that are related to the physical Pauli operators by a local unitary transformation, the... indicates additional terms which only involve τ operators, and the coefficients fall off exponentially with distance. This Hamiltonian manifestly contains an extensive number of localized integrals of motion or "l-bits" (the operators τ i, which all commute with the Hamiltonian). If the original Hamiltonian is perturbed, the l-bits get redefined, but the integrable structure survives.