Drain yams and mash well. Drain pineapple, reserving juice. Add juice to yams with sugar and butter. Beat well. Stir in pineapple and pecans. Spray a 9 in pan with non-stick spray. Bake in a 375 oven for 20 minutes. Sprinkle marshmallows on top, return to oven for 10 minutes more.
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