“…The content of the deque is given by a vertical scan line from bottom to top. The dashed edge (3,8) causes an error at vertex 7, since the content is ((6, 7), (6,8), (5,8), (4,7), (3,8), (3,7), (5, 7)) and (3,8) blocks the removal of (4, 7). Furthermore, at each vertex the incoming edges from predecessors must be removed before the outgoing edges to successors.…”