 On the other hand, to prove inexpressibility, i.e., that some sentence is not equivalent to any L-sentence, is harder. One way that sometimes works is to establish that \(L_1\) has some property that \(L_2\) lacks; then one might be able to conclude that \(L_1 \not\leq L_2\). Some properties that are typical of FO, but fail for most stronger logics, are: