[originally from svn r9453]
A black circle in a square indicates that that square is a corner, but
neither of the squares adjacent to it in the loop is also a corner.
-A while circle indicates that the square is a straight edge, but \e{at
+A white circle indicates that the square is a straight edge, but \e{at
least one} of the squares adjacent to it in the loop is a corner.
(In both cases, the clue only constrains the two squares adjacent