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29 December 1980–2 January 1983
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3 January 1983–July 1987
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July 1987–April 1989
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April 1989–April 1991
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September 1989–February 1993
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April 1991–September 1996
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September 1996–24 May 1998
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25 May 1998–7 September 2003
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8 September 2003–10 March 2006
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11 March 2006–31 October 2009
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1 November 2009–13 January 2013
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14 January 2013–present
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