Sokoban Level Normalizer

The motive of writing this tool is to normalize the MF8 Sokoban Competition level before being posted at the competition site. Basically, the normalizer will do the following:

1. Remove all the unreachable walls, boxes, etc.
2. Distinguish reachable and unreachable spaces, they are denoted by "-" and "_" respectively after normalization.
3. Add corner walls if needed.

The input level should be in XSB format. The normalizer supports input level using "-", "_" or " " for spaces, or any combination of them. The output normalized level will always use only "-" and "_" for spaces.

The normailizer will assume the input level is a proper level. In other words, it should has exactly one pusher and is enclosed by walls.