I'm looking at Bullett-Macdonald paper where they prove a reformulation of the Adem relations (
sciencedirect.com/science/article/pii/…) and I got terribly confused by something. At the start of the second page they say that if you take the product $y_1 \cdots y_n x_{n+1} \cdots x_{2n}$ where $y$'s are the exterior generators of $H^{\ast}(B\mathbb{Z}/p)$ and $x$'s are the polynomial generators.