Fill a large bowl with cold water. Trim and discard the greens tough stems, tear the leaves into pieces and wash them in the water. (If the greens are very dirty wash them in several changes of water.) Lift the greens from the water, and put them in a colander to drain. Set aside.
In a large Dutch oven or soup pot, cook the bacon over medium heat until browned and its fat has rendered, about 4 to 5 minutes. Add the scallions and cook for 2 minutes. Add the garlic and cook 1 minute more. Add the greens in batches, starting with the tougher greens, like kale, and ending with the more tender ones, like chard, stirring each batch as they wilt, before adding more. Add 1/2 cup water and cook, covered, stirring once or twice, until tender, about 10 minutes. Add the vinegar and season with salt and pepper to taste. Serve immediately.
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