A monoidal structure on the category of relative Hopf modules Vrije Universiteit Brussel
category $\Cc$, and assume that $A$ is also a coalgebra, with a not-necessarily
associative or unital left $B$-action. Then we can define a right $A$-action on the tensor
product of two relative Hopf modules, and this defines a monoidal structure on the category
of relative Hopf modules if and only if $A$ is a bialgebra in the category of ...