More than two statements combined by ~\leftrightarrow~ are ambiguous: .
~x_1 \leftrightarrow x_2 \leftrightarrow x_3 \leftrightarrow .. \leftrightarrow x_n may be meant as ~(((x_1 \leftrightarrow x_2) \leftrightarrow x_3) \leftrightarrow ..) \leftrightarrow x_n, .