You have to trace train tracks from "start" to "end" in a rectangular grid, respecting constraints:
- Each row and column must contain a given number of tracks.
- Some tracks are fixed and you must use them as they are. Their positions are given with the entry / exit directions (North / South / West / East). You can choose the entry.
- The exit direction of the start, and the entry direction of the end are fixed.
- The track may not cross itself.
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