diff --git a/days/day15.st b/days/day15.st
index 746ae99..db85459 100644
--- a/days/day15.st
+++ b/days/day15.st
@@ -4,21 +4,21 @@ Object subclass: Transaction [
     updates [^updates]
 
     putAll: o
-    [ o updates keysAndValuesDo: [
-        :pos :new | self at: pos put: new ]]
+    [ o updates keysAndValuesDo: [:pos :new | self at: pos put: new]]
 
     movedFrom: pos [ self at: pos put: $. ]
     at: pos put: new
     [ | old |
-      old := updates at: pos ifAbsent: [nil].
-      (old isNil or: [old = $.]) ifTrue: [^updates at: pos put: new].
+      old := updates at: pos ifAbsent: [$.].
+      old = $. ifTrue: [^updates at: pos put: new].
       (new = $. or: [old = new]) ifTrue: [^nil].
       self error: ('Contradictory transaction %1 <- %2: already %3'
                        % {pos . new . old}) ]
 
+    hasMovedFrom: pos [ ^(updates at: pos ifAbsent: [nil]) notNil ]
+
     performOn: grid
-    [ updates keysAndValuesDo: [
-        :pos :new | grid at: pos put: new ] ]
+    [ updates keysAndValuesDo: [:pos :new | grid at: pos put: new] ]
 ]
 
 Object subclass: Warehouse [
@@ -42,6 +42,7 @@ Object subclass: Warehouse [
 
     tryMoveIn: trans from: src by: off
     [ | blk |
+      "(trans hasMovedFrom: src) ifTrue: [^true]. -- just makes it slower"
       blk := grid at: src.
       blk = $. ifTrue: [^true].
       blk = $# ifTrue: [^false].
diff --git a/days/day16.st b/days/day16.st
new file mode 100644
index 0000000..de972e3
--- /dev/null
+++ b/days/day16.st
@@ -0,0 +1,103 @@
+Object subclass: Maze [
+    | grid start end bestScore
+      seen queue |
+
+    rows: r [ grid := Grid new rows: r. self findLandmarks ]
+
+    findLandmarks
+    [ grid allPosnsDo: [
+        :pos | | tile | tile := grid at: pos.
+        (tile = $S or: [tile = $E]) ifTrue: [
+            tile = $S ifTrue: [start := pos]
+                      ifFalse: [end := pos].
+            grid at: pos put: $.. ]] ]
+
+    bestPath
+    [ "seen[pos][dir] -> {score . Set(previous-states ...)}"
+      seen := grid collect: [:i | LookupTable new].
+      "queue: score -> {pos . facing}"
+      queue := SortedCollection new.
+
+      self enqueuePos: start dir: Posn right withScore: 0 fromState: nil.
+      self enqueuePos: start dir: Posn up withScore: 1000 fromState: nil.
+
+      [ queue isEmpty ] whileFalse: [
+          | entry state |
+          entry := queue removeFirst.
+          entry value letArrayInBlock: [
+              :pos :dir |
+              self stepPathAt: pos dir: dir score: entry key ]].
+
+      bestScore := FloatD infinity.
+      (seen at: end) do: [:it | bestScore := bestScore min: (it at: 1)].
+      ^bestScore ]
+
+    stepPathAt: oldPos dir: dir score: oldScore
+    [ | rots pos nextPos score |
+      rots := { dir rotateCw . dir rotateCcw }.
+      pos := oldPos.
+      score := oldScore.
+
+      [ nextPos := pos + dir.
+        (self isOpen: nextPos) ifFalse: [^nil].
+        score := score + 1.
+        self addPos: nextPos dir: dir withScore: score
+             fromState: { pos . dir }.
+        pos := nextPos.
+
+        rots do: [
+            :rot | | rotPos |
+            rotPos := pos + rot.
+            (self isOpen: rotPos) ifTrue: [
+                self enqueuePos: rotPos dir: rot
+                     withScore: score + 1001
+                     fromState: { pos . dir }.
+            ]]
+      ] repeat ]
+
+    addPos: pos dir: dir withScore: score fromState: oldState
+    [ | seenState oldScore cameFrom seenDirState |
+      seenDirState := seen at: pos.
+      seenState := seenDirState at: dir ifAbsent: [
+          seenDirState at: dir put: {FloatD infinity . Set new}
+      ].
+      oldScore := seenState at: 1.
+      cameFrom := seenState at: 2.
+
+      score > oldScore ifTrue: [^false].
+      score < oldScore ifTrue: [
+          seenState at: 1 put: score.
+          cameFrom empty ].
+      oldState ifNotNil: [ cameFrom add: oldState ].
+      ^score ~= oldScore ]
+
+    enqueuePos: pos dir: dir withScore: score fromState: oldState
+    [ | ok |
+      ok := self addPos: pos dir: dir withScore: score fromState: oldState.
+      ok ifTrue: [queue add: score -> {pos . dir}].
+      ^ok ]
+
+    isOpen: pos [ ^(grid at: pos) = $. ]
+    printOn: st [ st << grid ]
+
+    bestPathCount
+    [ | bestPathSet |
+      bestPathSet := Set new.
+      (seen at: end) keysAndValuesDo: [
+          :dir :i | (i at: 1) = bestScore ifTrue: [
+              self traceBackFrom: end dir: dir into: bestPathSet]].
+      ^bestPathSet size ]
+
+    traceBackFrom: pos dir: dir into: set
+    [ set add: pos.
+      grid at: pos put: $O.
+      (((seen at: pos) at: dir ifAbsent: [^nil]) at: 2) do: [
+          :nPos :nDir | self traceBackFrom: nPos dir: nDir into: set
+      ] asSpreader ]
+]
+
+AOC input: [ Maze new rows: stdin toLines asArray ];
+    part1: [ :maze | maze bestPath ];
+    part2: [ :maze | maze bestPathCount ];
+    result: [ :maze :part | part value: maze ];
+    finish.
diff --git a/days/utils.st b/days/utils.st
index 1b16079..879a351 100644
--- a/days/utils.st
+++ b/days/utils.st
@@ -212,7 +212,6 @@ Object extend [ chain [ ^MessageChain new pipelineValue: self ] ]
 
 Object subclass: Posn [
     | x y | x [^x] y [^y] x: n [x:=n] y: n [y:=n]
-    copy [ ^Posn x: x y: y ]
 
     + o [ ^Posn x: x + o x y: y + o y ]
     += o [ self x: x + o x; y: y + o y. ]