In a saucepan boil the cider until it is reduced to about 1/4 cup and let it cool. In a bowl whisk together well the pumpkin puree, the eggs, the oil, the brown sugar, the zest, and the reduced cider. Into the bowl sift together the flour, the baking powder, the salt, the baking soda, the mace, the cinnamon, and the cloves, add the walnuts, and stir the batter until it is just combined. Transfer the batter to a well-buttered 8 1/2-by 4 1/2-inch loaf pan and bake the bread in the middle of a preheated 350°F. oven for 1 hour, or until a tester comes out clean. Let the bread cool in the pan.