In a large, heavy pot cook the bacon until it has rendered most of its fat, about 5 to 6 minutes. Add the onions, garlic, salt, cayenne, and black pepper and cook until the onions are wilted, about 4 minutes. Add 1 quart of water, the beer, vinegar, and molasses and bring to a boil. Begin adding the greens in batches, pressing down with a wooden spoon to submerge them in the hot liquid and adding more as they wilt. When all of the greens have been added, reduce the heat to a simmer and cook the greens, partially covered and stirring occasionally, for about 1 to 1 1/2 hours or until tender.