  We consider percolation on interdependent locally treelike networks, recently
introduced by Buldyrev et al., Nature 464, 1025 (2010), and demonstrate that
the problem can be simplified conceptually by deleting all references to
cascades of failures. Such cascades do exist, but their explicit treatment just
complicates the theory -- which is a straightforward extension of the usual
epidemic spreading theory on a single network. Our method has the added
benefits that it is directly formulated in terms of an order parameter and its
modular structure can be easily extended to other problems, e.g. to any number
of interdependent networks, or to networks with dependency links.
