a similar orientation as the first openings 48 as with the first openings the second openings are uniformly spaced throughout the region bordered by the first transition zone intersecting the upper frame strand 22 and the second transition zone 56 intersecting the left and right frame strands 26 28 triangular shaped openings 62 are provided at spaced intervals between the second openings 50 and the frame strands 22 26 and 28 the third openings 52 are also of a rectangular shape positioned in a similar orientation as the first and second openings 48 50 the third openings are uniformly spaced throughout the region bordered by the second transition zone 56 and its intersection with the left and right frame strands 26 28 and the lower frame strand 24 as before triangular shaped openings 64 are provided at spaced intervals between the second rectangular openings 52 and the lower left and right frame shaped strands 24 26 and 28 an important aspect of the present invention is the relationship between the regional extent of the first large openings 48 to the intermediate sized second openings 50 to that of the smaller third openings 52 if the distance from the focal point 58 to the first transition zone 54 is u201c x u201d then the distance from the first transition zone to the second transition zone 56 ranges from about 0 2x.