Sprinkle yeast and sugar over 1/4 cup warm water. Stir to dissolve. Let stand
until foamy, about 10 minutes.
Melt chocolate and margarine with 1 1/4 cups water in a large bowl set over gently
simmering water. Stir until smooth. Remove from over water. Blend in molasses, vinegar
and salt. Mix in cereal. Let cool.
Grease a large bowl. Blend yeast into cereal mixture. Gradually stir in 2 cups
unbleached flour and rye flour. Turn dough out onto lightly floured surface and
knead until smooth and elastic, about 10 minutes, kneading in up to 3/4 cup more
unbleached flour if needed to form a workable dough. Add dough to prepared bowl,
turning to coat entire surface. Cover and let rise in warm area until doubled in
volume, about 2 hours.
Grease two loaf pans. Punch dough down. Turn out onto lightly floured surface
and let rest 3 minutes. Knead 3 minutes. Divide dough in half. Roll each into an
8 x 7-inch rectangle. Starting with long side, roll dough up into a cylinder. Tuck
ends under and pinch seam to seal. Place seam side down in prepared pans. Cover
and let rise in a warm place until doubled, about 1 1/2 hours.
Heat oven to 375 degrees F.
Bake until loaves sound hollow when tapped on the bottom, about 45 minutes.
Remove bread from pans. Let cool completely on a rack before serving.