try to optimize BFS solution

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

@@ -2,22 +2,25 @@ def tL = turn left end;
 def tR = turn right end;
 def tB = turn back end;
 def ifM = \p.\t.\e. b <- p; if b t e end;
-def DFSn = \n.
+// Returns true if blocked, false if it may be a viable path
+def DFSn : int -> cmd bool = \n.
   // say $ "DFSn at level " ++ format n;
   ifM (ishere "goal") {swap "path"; selfdestruct} {};
   if (n == 0) {} {
     ifM (ishere "path") {} {
       place "path";
-      tL; b <- blocked; if b {} {move; DFSn (n-1)};
-      tR; b <- blocked; if b {} {move; DFSn (n-1)};
-      tR; b <- blocked; if b {} {move; DFSn (n-1)};
-      tL; grab; return ()
+      tL; b <- blocked; bL <- if b {return true} {move; DFSn (n-1)};
+      tR; b <- blocked; bF <- if b {return true} {move; DFSn (n-1)};
+      tR; b <- blocked; bR <- if b {return true} {move; DFSn (n-1)};
+      tL; if (bL && bF && bR) {swap "rock"} {grab}; return ()
     }
   };
-  tB; move; tB
+  rockhere <- ishere "rock";
+  tB; move; tB;
+  return rockhere
 end;
 def startDFS = \n.
-  b <- blocked; if b {} {move; DFSn n}
+  b <- blocked; if b {} {move; DFSn n; return ()}
 end;
 def clear_rocks =
   ifM (ishere "rock") {
@@ -42,5 +45,6 @@ def for : int -> int -> (int -> cmd unit) -> cmd unit = \lo. \hi. \m.
 end;
 build {
   require 500 "rock"; require 500 "path";
+  log "hi";
   for 1 500 (\n. DFS n);
 }