Grammars $𝐴$ and $𝐵$ are sequentially unambiguous if the followlast set of $𝐴$ is disjoint from the first set of $𝐵$.

We can understand this intuitively by characterizing the behavior of a left-to-right parser of $𝐴\otimes 𝐵$. First it searches for a parse of $𝐴$, then upon finding a character that is not in $\text{𝖥𝖫𝖺𝗌𝗍}(𝐴)$ it may begin trying search for $𝐵$.

That is, there is a unique boundary between the $𝐴$-parse and the $𝐵$-parse.