 We use the same concept of an L-structure (or equivalently, an L-model) as in first order logic. That is, an L-structure \(\mm\) has a domain M, which is any non-empty set, an interpretation \(c^\mm \in M\) of any constant symbol c of L, an interpretation \(R^\mm \subseteq M^n\) of any n-ary relation symbol R of L, as well as an interpretation \(H^\mm : M^n \to M\) of any n-ary function symbol H of L.