Bring the water to a boil in a soup pot. Add the lentils, reduce the heat, and simmer for 20 minutes. Then add the onion, celery and carrot and cook another 10 minutes, uncovered, until the lentils are almost tender. Heat the oil in a frying pan, then add the Swiss chard or spinach, and sauté for a few minutes. Transfer the greens to the soup. Season with salt, pepper, cumin, allspice and lemon juice, adjusting the seasonings to your taste. To serve, ladle into soup bowls top with broken-up pieces of toasted pita bread, a dollop of yogurt and a sprinkle of sumac.