IV.D241 — Tower-natural transformation

A tower-natural transformation eta: F => G between sector functors F, G: tau^1 -> tau^3|_{T^2} is a family {eta_n: F[n] -> G[n]} in the boundary holonomy algebra H_partial satisfying eta_{n+1} circ Phi^F = Phi^G circ eta_n for all tower levels n >= 1, encoding compatibility with propagation.