I was reading the paper Computing Small Search Numbers in Linear Time by Bodlaender et. al. and I noticed a notation that I had never seen before. On page three it says:
$X$ is nice if $|X_1| = 1$ and $\forall_{2 \leq i \leq |X|} |(X_i \Leftrightarrow X_{i-1}) \cup (X_{i-1} \Leftrightarrow X_i)| = 1 $
What does the logical equivalence symbol mean when it is used between sets? Does this indicate a bijection?