In medium-size bowl, blend prunes, Sweet ‘ N Low, vanilla, and lemon juice. In small saucepan, soften gelatin in cold water, placing over low heat to dissolve. In separate medium-size bowl, beat egg whites until frothy. Add dissolved gelatin and beat until stiff; fold into prune mixture. Spoon lightly into sherbet glasses. Sprinkle walnuts on top. Chill thoroughly.