 An <formula> <unk> for a subdivision rule <formula> is a 2 @-@ dimensional CW complex <formula> which is the union of its closed 2 @-@ cells , together with a continuous cellular map <formula> whose restriction to each open cell is a homeomorphism . We can subdivide <formula> into a complex <formula> by requiring that the induced map <formula> restricts to a homeomorphism onto each open cell . <formula> is again an <formula> <unk> with map <formula> . By repeating this process , we obtain a sequence of subdivided <formula> <unk> <formula> with maps <formula> . 
