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Pankaj Agarwal has been supported by National S ien e Foundation Grant CCR-91-06514. Mi ha Sharir has been supported by OÆ e of Naval Resear h Grant N00014-90-J-1284, by National S ien e Foundation Grant CCR-89-01484, and by grants from the U.S.{Israeli Binational S ien e Foundation, the G.I.F. | the German Israeli Foundation for S ienti Resear h and Development, and the Fund for Basi Resear h administered by the Israeli A ademy of S ien es. Sivan Toledo has been supported by the Spe ial Interdis iplinary Program at Tel-Aviv University. y Department of Computer S ien e, Duke University. z Courant Institute of Mathemati al S ien es, New York University, and S hool of Mathemati al S ien es, Tel Aviv University. x S hool of Mathemati al S ien es, Tel Aviv University; Current address: Laboratory for Computer S ien e, Massa husetts Institute of Te hnology.