void AddBoxColliderTo(GameObject coll_go, ref BoxCollider2D bc, Vector2 bcBegin, int bcWidth)
{
bc.offset = bcBegin + new Vector2(bcWidth * VOXEL_SIZE * 0.5f, VOXEL_SIZE * 0.5f);
bc.size = new Vector2(bcWidth * VOXEL_SIZE, VOXEL_SIZE);
//Debug.Log (bc.offset.ToString() + " sz=" + bc.size.ToString());
bc = null; // forget it, leave hanging
}