Implement a 4-variable Boolean function f(x1, x2, x3, X4) = E(2,4,5,7,8,9,11,13,15) using a single 4-to-1 multiplexer and minimal number of two-input NOR gates. Use xi and x2 as select input lines in the multiplexer. Use only variables x1, x2, x3, X4 as inputs (not their negated forms).