Blast Kills 10 in Iraq
Iraqi National Assembly, bowing to strong pressure from United Nations, votes to cancel last-minute rule change that all but guaranteed passage of new constitution in Oct 15 referendum; Sunni Arab leaders had accused Shiite and Kurdish leaders of trying to rig referendum; Shiite leaders who vote to cancel rule change say they reserve right to challenge results if constitution fails in vote; their fear is that insurgents might manipulate vote by attacking Shiites to keep them away from polls; dispute sharpens sectarian divisions over constitution, and uneasy resolution seems to open door to further dissent, especially if violence worsens; in Washington, State Dept spokesman calls reversal of controversial rule 'a positive step'; photos (M)