don't need to worry about SPR A, since to_base36 will never produce capital letters

src/game.rs

@@ -360,9 +360,8 @@ impl Game {
     ///      if current tile IDs are [0, 1, 2] the result will be `3`
     #[inline]
     pub fn new_tile_id(&self) -> String {
-        // don't allow `0` - this is a reserved ID
-        // todo new_sprite_id will need to avoid A too
         let mut ids = self.tile_ids();
+        // don't allow 0 - this is a reserved ID for an implicit background tile
         ids.push("0".to_string());
         new_unique_id(ids)
     }