EXHIBIT A
JOINT FILING AGREEMENT
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In accordance with Rule 13d-1(k)(l) under the Securities Exchange Act of 1934,
as amended, the undersigned hereby agree to the joint filing of the attached
Schedule 13G, and any and all amendments thereto, and expressly authorize
AMVESCAP PLC, as the ultimate parent company of each of its undersigned
subsidiaries, to file such Schedule 13G, and any and all amendments thereto, on
behalf of each of them.
Dated: May 9, 2007 AMVESCAP PLC
By: /s/ Xxxx Xxxxxxxx
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Name: Xxxx Xxxxxxxx
Title: Global Compliance Director
PowerShares Capital Management LLC
By: /s/ Xxxxx Xxxxxxxxx
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Name: Xxxxx Xxxxxxxxx
Title: General Counsel & COO
