Time-stamp: </home/wrf/git/all/c/overprop/db/Notes, Thu, 15 Nov 2018, 20:20:00 EST, w540, wrf, W. Randolph Franklin, http://wrfranklin.org/>

File format:

- Each map is a planar graph with polygons (faces) separated by polylines (poly edges, borders).

- Each polygon has a positive integer id; the outside is #0.

- The only explicit element type is the polyline.

- Implicitly, the map must be topologically valid.   The end point of a polyline must exactly coincide with the end points of one, or usually two, other polylines.

- The map may have multiple components, which may be nested.

- The file is a list of polylines.

- Each polyline has a header line followed by a list of 2D float points.

- The header line has 6 ints.

  #1, #3, #4 we ignore.

  #2 is the number of points in the polyline (including the end points).

  #5 and #6 are the ids of the left and right neighboring polygons.

- The file's optional last line has 6 negative ints.
