Fig. 1

a Concatenation of any two neighboring special nodes s 1 and s 2(both from a). The nodes s 1 and s 2 are replaced with one special node s 1 s 2 (the concatenated sequence of the sequences of two initial special nodes). Similarly for (b). b Removal of a special node. Large point is an a-special node s and the resulting combined edge is marked (a). Similarly for (b)