11 b first as shown in fig1 a and 10b an opening spreader 51 having three hooks 51 a which are disposed at an interval of 180 degrees is disposed in such a manner that the respective hooks 51 a are engaged with respective three line elements 23 at middle positions between the respective converging sections 24 then by pulling the hooks 51 a in the direction of arrow a in fig1 b the three line elements 23 are spread apart at the middle positions of the converging sections 24 as shown in fig1 a and 11b thus when the hooks 51 a have been withdrawn the unit structure 25 which consists of a plurality of basic units 22 each of which is a combination of two triangular pyramids having a common bottom surface 22 d and apexes 22 a disposed in opposite directions the basic units 22 being arranged continuously in vertical direction is formed the converging section 24 of this unit structure 25 is shown in the perspective view of fig1 a plurality of the unit structures 25 are disposed in such a manner that as shown in fig1 horizontally adjacent ones of the unit structures 25 come into contact with each other so that the adjacent one of the unit structures 25 are in a position to form the converging section 26 of the three line elements 23 at each apex 22 c of the common bottom surface 22 d of the two triangular.