How to define a bijection function from A\B to B/A

Question

how to construct a bijection from A\B to B\A. I was thinking of defining a function that gets an element that is contained in the set A but not in B and "makes" the element to be contained in B but not in a.
but I don't know to write such a function

Answer 1

There is no such bijection in general. For instance, if $B\subset A$, then $A\setminus B$ has elements, but $B\setminus A$ is empty.