Horizontal lines have equations of the form \(y = c\). Draw the graph of \(x = 3\). Mark some points on a grid which have an \(x\)-coordinate of 3, such as (3, 0), (3, 1), (3, -2). The points lie ...

We present in Section 7.3 a short proof of Thomassen [Th97c] which is based on two other important results in the Robertson-Seymour theory, namely Theorem 7.2.1 (well-quasi-ordering of graphs of ...