“…When M 4 is a toric surface, the general gluing construction, illustrated in (2.4) or (2.14), leads to various extensions of W -algebras for g = Lie(G) of the same type as the original "gauge" group G. Although this general gluing construction is supported by a number of direct calculations [14,16,17,32], which build on the AGT conjecture [25] for M 4 = R 4 1, 2 , it may seem puzzling. Indeed, when G = U (N ) and M 4 is a 4-manifold with a linear plumbing, as in Figure 1:…”