 Let \(\bA\) and \(\bB\) be \(L\)-algebras. \(\bA\) is a subalgebra of \(\bB\) if (1) \(A \subseteq B\), (2) the interpretations of the 0-ary symbols of \(L\) in \(\bB\) belong to \(A\) and \(A\) is closed under the functions of \(\bB\) that interpret the non 0-ary symbols, and (3) the interpretations of the 0-ary symbols in \(\bA\) coincide with their interpretations in \(\bB\) and the interpretations on \(\bA\) of the other symbols in \(L\) are the restrictions to \(\bA\) of their interpretations in \(\bB\).