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This paper has been withdrawn by the author. Proofs of two propositions need more details.

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This paper has been withdrawn by the authors due to substantial changes.

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This paper has been withdrawn by the authors, due to the requirement of the Journal where a modified version will be published.

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This paper is withdrawn by the author. See math.GT/9811093 for replacement.

This paper has been withdrawn by the author due to similarity to Author's other paper

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This paper has been withdrawn by the author, as it is now incorporated in 0901.4506 (v4)

This paper has been withdrawn by the author, because it is now part of an enlarged version entitled "Time functions as utilities" arXiv:0909.0890