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Minor improvement to initial loop generation.

[originally from svn r6292]

--- a/loopy.c

+++ b/loopy.c

@@ -764,7 +764,16 @@

         square = (struct square *)index234(lightable_squares_sorted, 0);

         assert(square);

-        if (square->score <= 0)

+	/*

+	 * We never want to _decrease_ the loop's perimeter. Making

+	 * moves that leave the perimeter the same is occasionally

+	 * useful: if it were _never_ done then the user would be

+	 * able to deduce illicitly that any degree-zero vertex was

+	 * on the outside of the loop. So we do it sometimes but

+	 * not always.

+	 */

+        if (square->score < 0 || (square->score == 0 &&

+				  random_upto(rs, 2) == 0))

             break;

         print_tree(lightable_squares_sorted);