When normal-ordering a product of fermionic operators, each swap of adjacent operators may generate new terms via the canonical anti-commutation relations (CAR): {aᵢ, a†ⱼ} = aᵢ a†ⱼ + a†ⱼ aᵢ = δᵢⱼ The ICombiningAlgebra interface abstracts this: Combine takes two operators and returns the terms generated by swapping them. FermionicAlgebra implements the fermionic CAR. Other algebras (bosonic, anyonic) could implement the same interface with different commutation relations.