Fig. 3

Two paths \(P=(0,\ldots ,w)\) and \(Q=(0,\ldots ,a,b)\). In every step, the algorithm extends the path ending in the smaller mass. In the next step, P is extended by an edge \((w,w')\). Dotted lines point towards masses on the real line (bottom) that are in \(\text{TSe}(a,b),M)\), respectively \(\text{TSe}((w,w'),M)\) (the complementary masses are omitted). As shown in this example, these two sets can have a non-empty intersection