Mix together the yeast, sugar, salt, and flour in a large mixing bowl. Make a well in the center and add the warm water. Mix to a soft dough. Knead on a floured board for ten minutes. Return to the bowl, cover with a damp cloth and put in a warm place to rise for one hour, or until it doubles in size. Knead lightly and again leave to rise until it doubles in size. Roll into a long rope about 1 ?½ inches (4cm) in diameter. Cut into 1-inch (2 ?½-cm) pieces and roll into ovals 4 inches (10 cm) long. Dust one side with flour and double over. Allow to rise for 15 minutes or until double in size. Cook in a steamer for 10-15 minutes, until light and spongy. These may be eaten plain or stuffed with sweet or savoury filling. See below.