Let M, N be L-structures. M is a substructure of N if M ⊆ N and for every constant symbol c, relation symbol R and function symbol f of L, we haveNote that since M is an L-structure, the condition on function symbols implies that M, as a subset of N , is closed under f N . We also say that N is an extension of M. The substructure is proper if its domain is a proper subset of the domain of the extension, in which case we write M < N . Note that M ≤ N ⇔ M < N ∨ M = N , whence this defines a partial order.If a subset S of N is closed under the interpretation of all constant and function symbols of N , then we have a substructure N ↾ S of N , called the restriction of N to S, on the domain S defined by c N ↾ S = c N , R N ↾ S = R N ∩ S arity(R) and f N ↾ S = f N ∩ S arity(f )+1 , for all constant, relation and function symbols, c, R, f , respectively.An embedding f : M → N from an L-structure M to an L-structure N is a function f : M → N , such that for each atomic L-formula φ( x) and for each m ∈ M, we have