idea for shortest path solution

data/scenarios/Challenges/Mazes/_shortest_path/shortest_path_sol.sw

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+// Idea: keep a fringe of locations marked with some entity (say,
+// flowers), such that at every step a location has a flower iff it is
+// exactly n steps away.  To expand it, do DFS (marking visited spots with rocks),
+// and when finding flowers, mark adjacent empty spots with some third entity;
+// then do a second pass to get rid of flowers; then a third pass to turn the
+// third entity into flowers.