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Suppose you want to build a double half-adder that operates on four input bits instead of two. This adder takes four bits x1, x2, x3, and x4 as input, and outputs three bits y1, y2, and y3 such that y3y2y1 = x4x3 + x2x1. Give boolean formulas for each of the bits y1, y2 and y3. You may use AND (), OR (), NOT () and XOR () clauses in your responses.