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Place mashed avocado into bowl. Add 1 tbsp lemon juice. Add cayenne pepper and salt. Combine well. Place shrimp in another bowl. Add mayonnaise, chopped coriander, and salt. Stir well. Spoon the avocado mix onto 2 slices of bread, and spread well. Heap shrimp mixture on top. Press down lightly with spoon. Place the other slices on top. Slice in half.