  We show that, assuming the Axiom of Determinacy, every non-selfdual Wadge
class can be constructed by starting with those of level $\omega_1$ (that is,
the ones that are closed under Borel preimages) and iteratively applying the
operations of expansion and separated differences. The proof is essentially due
to Louveau, and it yields at the same time a new proof of a theorem of Van
Wesep (namely, that every non-selfdual Wadge class can be expressed as the
result of a Hausdorff operation applied to the open sets). The exposition is
self-contained, except for facts from classical descriptive set theory.
