 1 . A finite 2 @-@ dimensional CW complex <formula> , called the subdivision complex , with a fixed cell structure such that <formula> is the union of its closed 2 @-@ cells . We assume that for each closed 2 @-@ cell <formula> of <formula> there is a CW structure <formula> on a closed 2 @-@ disk such that <formula> has at least two vertices , the vertices and edges of <formula> are contained in <formula> , and the characteristic map <formula> which maps onto <formula> restricts to a homeomorphism onto each open cell . 
