int rx = t.tx - c.tx, ry = c.ty - t.ty; return Tile_coord(c.tx + ry, c.ty + rx, t.tz);
Tile_coord r = Rotate90r(obj->get_tile(), c);   r.tx = (r.tx + ytiles + c_num_tiles)%c_num_tiles; r.ty = (r.ty + c_num_tiles)%c_num_tiles; return r;
inline Tile_coord Rotate90r	(	Tile_coord t,			// Tile to move.	Tile_coord c			// Center to rotate around.	)	{					// Get cart. coords. rel. to center.	int rx = t.tx - c.tx, ry = c.ty - t.ty;	return Tile_coord(c.tx + ry, c.ty + rx, t.tz);	}