In a large bowl, stir together sugar and tapioca. Add rhubarb, raspberries, and lemon juice; mix gently but thoroughly. Let stand 15 minutes or up to 1 hour to soften tapioca; mix gently several times.

On a lightly floured board, roll half the pastry into a 12-inch-diameter round; ease into a 9-inch pie pan. Fill with rhubarb mixture. On floured board, roll remaining pastry into a 10-inch-diameter square. With a pastry wheel or knife, cut into 8 equal strips. Arrange strips on top of pie in lattice pattern; trim off as they lap over rim. Fold bottom crust over lattice, flush with pan rim; flute to seal.

Set pie in a foil-lined 10- by 15-inch pan (pie bubbles as it cooks). Bake in a 400 oven on the lowest rack until pastry is golden brown and filling is bubbly, 40 to 50 minutes. If rim begins to darken excessively before filling bubbles, drape with strips of foil.

Serve warm or at room temperature. If made ahead, let cool, then cover loosely and store at room temperature up until next day. Cut into wedges.

Per serving: 412 cal.; 3.8g protein; 18 g fat; 619 Carbo.; 64 mg sodium; 0 mg chol.

Raspberry vinegar pastry. In a bowl, combine 2 1/4 cups all-purpose flour and 1/4 teaspoon salt. With a pastry blender or your fingers, cut or rub in 3/4 Cup solid vegetable shortening, butter, or margarine until fine crumbs form. Sprinkle 2 tablespoons raspberry vinegar and 3 to 5 tablespoons cold water over crumbs. Stir with a fork until dough holds together. Divide dough in half and pat each portion into a smooth flat round. Use or, if made ahead, seal in plastic wrap and chill up to 3 days.