code: public boolean canTraverse (Object traverser, int tx, int ty)
      {
          if (_covered[index(tx, ty)]) {
              return false;
          }
  
          // null base or impassable base kills traversal
          BaseTile base = getBaseTile(tx, ty);
          if ((base == null) || !base.isPassable()) {
              return false;
          }
  
          // fringe can only kill traversal if it is present
          BaseTile fringe = getFringeTile(tx, ty);
          return (fringe == null) || fringe.isPassable();
      }
docs: Returns true if the specified traverser can traverse the specified
  tile (which is assumed to be in the bounds of this scene block).