  Consider a push-out diagram of spaces C <-- A --> B, construct the homotopy
push-out, and then the homotopy pull-back of the diagram one gets by forgetting
the initial object A. We compare the difference between A and this homotopy
pull-back. This difference is measured in terms of the homotopy fibers of the
original maps. Restricting our attention to the connectivity of these maps, we
recover the classical Blakers-Massey Theorem.
