
1.8. FRONTIERS IN PLANARITY 31
Of course, if the clockwise d ir ec ti on of the adjacency list of each vertex of a block is
independently chosen, this would not necessarily produce a planar embedding. However, it
is not difficult to devise some convention to transfer the orientation of the adjacency list
of one vertex to the adjacency lists of the neighboring vertices. For example, it may be
prescribed that if in the adjacency list of vertex v
i
the list item of vertex v
j
uses ref1 as
next and ref2 as prev, the same choice is made for the list item of v
i
in the adjacency list
of v
j
(in [BM99, BM04] a less intuitive, but more practical, convention is adopted). ...