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House members were moved to tears on Wednesday when Representative Gabrielle Giffords handed in her resignation because, she said, she needed more time to heal from the grievous gunshot wound she suffered last year at the hands of a would-be assassin. The rarest of Congressional events — a display of genuinely bipartisan tributes — was the parting gift from her colleagues along with unanimous approval of Ms. Giffords’s bill to fight drug trafficking along the Mexican border.