  We describe in a simple setting how to extract a unitary braided fusion
category from a collection of superselection sectors of a two-dimensional
quantum spin system, corresponding to abelian anyons. The structure of the
unitary braided fusion category is given by F and R-symbols, which describe
fusion and braiding of the anyons. We then construct the double semion state in
infinite volume and extract the unitary braided fusion category describing its
semion, anti-semion, and bound state excitations. We verify that this category
corresponds to the representation category of the twisted quantum double of
Z_2.
