Prefix as a Substring 2

Given $3$ strings $S_1$, $S_2$ and $X$, find the number of distinct [ordered pairs](https://en.wikipedia.org/wiki/Ordered_pair) of strings $(P,Q)$ such that : - String $P + Q$ is a substring of $X$. - String $P$ is some prefix of $S_1$ (possibly empty). - String $Q$ is some prefix of $S_2$ (possibly empty). A **substring** of a string is a contiguous subsequence of that string. For example, "