The following is a talk that I gave in at the event on "Quantum field theory with boundaries, impurities, and defects" at the Isaac Newton Institute in Cambridge. It is worth remarking that the contents of this work has changed a bit since then: the `magnetic symmetry' mentioned here is almost always trivial, and so this procedure seems to capture FLAT gauge theories, not dynamical. See my publications page for more details in the associated paper, which is now on arXiv!