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 detour calculation this is a bit worse, because not the whole bent network is
 moved, only the tiny veryical line segment in the middle between b and c so
 the drawing on c becomes something like d.
+
+<h4> 4.1.8. Geometry: blocking spiral </h4>
+<p>
+In case of the following spiral (blue net), the low level algorithm is unable
+to find the otherwise clearly available path for N1. The reason is that this
+is the "one endpoint in the hull, other endpoint outside of the hull" case,
+which means a clear sight of line is searched between the inner endpoint
+and the naked hull.
+<p>
+<div class="img"><center>
+<p><img src="img/ex_sprial.png">
+<p>Figure 4/8. Spiral can block the low level hull based algorithm.
+</center></div>
+<p>
+Note: the problem is not present if the blue net is split into two sections
+anywhere on the east or south wall (to the right from the conflict), because
+each section is searched separately. That is, the inner part of the spiral is
+in conflict first, which can be detoured through the second segment, then the
+second segment conflict is resolved without considering the first segment, then
+the first segment is conflicted again, etc. So the blocker is not that the
+final path is a spiral.
+