Crab, Jicama and Tomato Ceviche   3 medium tomatoes 4-1/2 cups of crab 3 medium cucumbers 1 medium onion 3 Serrano chiles 1 cup chopped cilantro 1 jicama, peeled and diced 9-10 green lemons Dice first 7 ingredients and mix them well. Squeeze juice from lemons and add to mixture. Add salt to taste. Chill. Serve cold with crackers, or on top of tostadas.