b-Invariant Edges in Bricks

The evolution of the approach to the Conjecture 13.2, as explained in Chapter 13, startedwith the observation that if a removable edge in a brick is not b-invariant, then that brick must have a nontrivial separating cut or, equivalently, that any removable edge in a solid brick is b-invariant. This led to the notion of robust cuts and to the consideration of the use of contractions with respect to a robust cut as an inductive tool. All this has been described in Chapter 13.