Almost there

sources/scripts/pos.js

@@ -0,0 +1,15 @@
+function Pos(x,y)
+{
+  this.x = x;
+  this.y = y;
+
+  this.toString = function()
+  {
+    return x+","+y;
+  }
+
+  this.sub = function(pos2)
+  {
+    return new Pos(this.x - pos2.x,this.y - pos2.y)
+  }
+}