If using frozen rhubarb, thaw but do not drain. In a large bowl stir together sugar and tapioca. Add fruit, lemon juice, vanilla, and mace or nutmeg. Gently toss to coat fruit. Let mixture stand 15 minutes, stirring once. Meanwhile, preheat oven to 375 degree F. On a lightly floured surface roll out one ball of pastry to a 12-inch circle; carefully transfer pastry to a 9-inch pie plate. trim pastry to 1/2 inch beyond edge of pie plate. Transfer fruit mixture to pastry-lined pie plate.
For lattice top, roll out remaining dough as above. Cut into 1/2-inch-wide strips. Weave strips on top of filling to make a lattice. Press ends of strips into rim of bottom crust. Fold bottom pastry over strips; seal and crimp edge. Cover edge of pie with foil. Bake for 25 minutes. Remove foil. Bake for 20 to 25 minutes more or until top is golden brown and juices are bubbly. Cool. Makes 8 servings.
Pastry for Lattice-Top Pie
Stir together flour and salt. Cut in shortening until pieces are the size of small peas. Using a total of 6 to 7 tablespoons cold water, sprinkle 1 tablespoon water over part of mixture. Toss with a fork. Push to side of bowl; repeat, until all is moistened. Divide in half; form each half into a ball.