1. Combine water and sweetener in a saucepan; bring to a boil. Add orange rind; reduce heat, and simmer 5 minutes. Strain mixture, discarding rind. Cool completely.
2. Add grapefruit juice and lemon juice to water mixture; stir well. Pour into a 9-inch square baking pan; cover and freeze until firm.
3. Break frozen mixture into pieces. Add frozen mixture to a blender or food processor, in batches, and process until smooth but not thawed. Serve immediately.
carbo rating: 6