[Math] Matrix product notation

Question

My lecturer has used some notation that I've never seen before: it is a (matrix) product symbol with a left-to-right arrow over the top. Does anybody know what this means?

Thanks in advance.

Edit: It looks like this:

Answer 1

Matrix multiplication is not commutative, so (maybe) that symbol means that you are considering the product adding the next matrix to the right: $$ X_1 X_2 \cdots X_k. $$ Anyway, I am just guessing, I've never seen it before!